Measure the way bakers do
A cup of all-purpose flour is supposed to weigh about four and a quarter ounces, or 120 grams. Scoop the cup straight into the bag and you can pack in 150 grams without trying, which is enough extra flour to turn a tender cake dry. A digital scale costs about fifteen dollars and ends the problem. If you must use cups, fluff the flour, spoon it into the cup and sweep the top level with a knife. Read liquid measuring cups at eye level.
Know your oven
Home ovens routinely run twenty-five degrees or more away from what the dial says. Hang an inexpensive oven thermometer from the center rack and adjust accordingly. Preheat for a full fifteen to twenty minutes, since many ovens beep well before they are truly hot. Bake on the middle rack unless told otherwise, and keep the door shut for the first two-thirds of the baking time, since a rush of cool air can collapse a rising cake. Dark metal and glass pans brown faster than shiny ones, so lower the heat by twenty-five degrees when you use them.
Temperature of ingredients
When a recipe calls for room-temperature butter, it means about 65 degrees: cool to the touch and soft enough to take a fingerprint without looking greasy. Butter at that stage traps air when it is beaten with sugar, and that air is much of the lift in a cake or cookie. Cold eggs can make the creamed butter seize and curdle, so warm them in a bowl of warm tap water for five minutes.
Leaveners and mixing
Baking soda needs an acid in the batter, such as buttermilk, yogurt, brown sugar or lemon juice, in order to work. Baking powder carries its own acid. They are not interchangeable spoon for spoon. Both fade after six months to a year once opened. Fresh powder fizzes hard in hot water, and fresh soda fizzes in vinegar.
Once the flour goes in, stir only until the dry streaks disappear. More mixing develops gluten, which is welcome in bread and produces tough, tunneled muffins.
Knowing when it is done
Cakes and quick breads are done when a toothpick from the center comes out clean or with a few dry crumbs. Cookies should come out while the centers still look slightly underdone, because they finish setting on the hot sheet.
Tip Weigh and set out every ingredient before you start mixing. The forgotten teaspoon of baking powder is nearly always discovered after the pan is already in the oven.
