There’s no bigger baking disappointment than realising you forgot to soften the butter. In the interest of eating cake as soon as possible, you might start to wonder: Does this butter really need to be room temperature — and if not, can I just melt it?
First things first: Never bake with melted butter unless the recipe specifically calls for it. Although we think of it as pure, delicious fat, butter is actually an emulsion of fat, milk solids (protein), and water. It only mixes evenly into doughs and batters when fully emulsified, and melting breaks that emulsion. This is why subbing browned butter for softened butter can produce flat, greasy baked goods. It’s like using vinaigrette instead of mayonnaise and wondering why your spinach-artichoke dip turned out so oily.
So melted butter is out — but cold butter is actually fine, at least in some applications. It depends on what you’re combining it with, and how. For a standard base of creamed butter and sugar, I usually chuck fridge-cold butter straight into the stand mixer and bash it around for a couple minutes to warm it up. Once it’s nice and soft, I add the sugar and proceed with the recipe. (The same strategy also works with food processors and handheld mixers, though the latter may take a bit longer.) But incorporating cold butter into softer, squishier substances — like brioche dough, cream cheese, or meringue — doesn’t work at all. You’ll end up with tiny globs of hard butter that no amount of mixing will disperse.
The basic rule of thumb: If a recipe starts with butter and sugar, cold is perfectly fine — just be sure to beat the butter by itself for a bit to warm it up. If you’re adding butter to something else, it absolutely must be room temperature. That doesn’t have to throw a wrench in your baking schedule, though. I soften butter by heating whole sticks in the microwave for 5-10 seconds per side; it works great and takes less than a minute. Alternatively, just slice butter into pieces and leave it out. It’ll be ready to go in 10-20 minutes, depending on the temperature of your kitchen, which is plenty of time to get the rest of your mise en place together.