Take one cookie recipe, make it twice, and change nothing except the state of the butter. Softened butter, beaten with the sugar, gives you a cookie that holds its height and has a cakey, tender bite. Melted butter, stirred in, gives you a flatter cookie that is chewier and denser and spreads further on the sheet.
Same ingredients. Same oven. Completely different cookie.
The reason is that softened butter and melted butter are not two temperatures of one ingredient. They are two different jobs. One of them can hold air and the other cannot, and everything downstream follows from that.
Softened butter builds a structure. Melted butter cannot.
Butter at room temperature is a solid fat that is soft enough to be worked but still solid. When you beat it with sugar, the sharp edges of the sugar crystals cut into the fat and open thousands of tiny air pockets, and the fat holds them there. That is the whole point of the step, and it is what creaming butter and sugar is actually doing.
Those pockets are the seed structure of the finished cookie. In the oven, the leavening gas and the steam expand into pockets that already exist rather than having to make new ones, and the cookie rises around them.
Melted butter is a liquid, and a liquid cannot hold an air pocket open. Beat melted butter and sugar together for ten minutes and you will have warm sweet liquid, not an aerated cream. So a melted-butter dough enters the oven with no seed structure at all, and everything it does afterwards has to come from steam and leavening pushing through a dough that has nothing waiting for it.
That is the single mechanism. Everything below is a consequence.
What each consequence looks like on the tray
Spread
Melted-butter cookies spread more, for two reasons that stack.
First, the dough is softer going in, because the fat is already liquid rather than waiting to melt. It begins flowing the moment it warms, before the structure has set.
Second, there is no aerated framework holding it up from the inside. The dough is being asked to set from a puddle instead of from a foam.
If your cookies came out flatter and greasier than expected, the butter's state is one of the first things worth checking. The full read on that failure, with the other causes separated out, is on why cookies come out flat and greasy, and the opposite problem is on why cookies did not spread at all.
Chew versus cake
This is the part most people are actually chasing.
A creamed dough is full of small air cells, which is a cake structure in miniature. It bakes up lighter and more tender and reads as cakey. That is exactly the finish described in why cookies come out cakey rather than chewy, and if that is not what you wanted, the butter is a lever.
A melted-butter dough has no air cells, so what sets is a denser continuous crumb. Denser reads as chewier. Chew is largely the absence of aeration, which is why melted butter is the standard move in recipes aiming for it.
Hydration, and why the flour behaves differently
Liquid butter wets flour differently from solid butter. Solid fat coats flour particles and keeps water away from the protein, which limits gluten development. Melted butter mixes more freely with the liquid in the dough, so more of the flour's protein gets hydrated and a little more gluten forms.
A modest amount of gluten in a cookie is not a fault. It contributes to the pull that people call chew. It is the same underlying property that makes overworking a cake batter a problem and makes it a virtue in bread.
The middle state nobody names
There is a third option that recipes rarely spell out: melted butter that has been allowed to cool back to room temperature before use.
That dough behaves closer to melted than to softened, because once butter has been melted the crystal structure that let it hold air is gone and simply cooling it down does not restore it. Butter that has been melted and cooled is thicker and easier to handle, and it will not hold air the way butter that was never melted does.
This matters practically. If you melted the butter to speed things up and then chilled the dough hoping to get a creamed result, you will not get one. You will get a firmer version of the melted-butter cookie, which may still be exactly what you want, but it is a different cookie.
How to choose
Read the recipe's target rather than the temperature line.
Softened, beaten with the sugar, when you want height, tenderness, a lighter crumb, or a cookie that keeps its shape. Also whenever the recipe leans on creaming for its rise rather than on chemical leavening alone.
Melted, stirred in, when you want spread, density and chew, and when the recipe's own texture note points there.
And do not substitute one for the other silently. Swapping butter states is not like swapping a brand of vanilla. It changes the structure the recipe was written around, and the result will differ in ways the ingredient list does not predict.
The one thing to avoid
Half-melted butter, the state you get from being impatient with a microwave, is the worst of both. Part of it is liquid, so it will not cream properly, and part of it is solid, so the dough is unevenly hydrated. The result is unpredictable rather than merely different.
If you have overshot and the butter has gone liquid, the honest move is to accept it, treat the recipe as a melted-butter recipe, and expect a flatter, chewier cookie. That is a better outcome than trying to cool it back into a state it can no longer occupy.
What this page does not give you
No temperature in degrees for "softened". The figure quoted for room-temperature butter varies with kitchen temperature, brand and the recipe's own tolerance, and the practical test is better than any number: the butter should take the print of a finger with slight pressure and hold its shape, rather than dent easily or feel greasy.
No timing for how long to cream. That depends on your mixer, your bowl and your butter, and a number here would be repeated by other pages without ever having been tested. The signal to watch for is the mixture going noticeably paler and lighter, which is the air arriving.
Allergen note. Butter is a dairy ingredient. This page explains what butter is doing mechanically and does not offer substitutions, and any swap for a plant-based fat behaves differently in exactly the ways described above, because the water and fat content differ.