In a mixing bowl with a paddle attachment, mix flour, sugars, cornstarch, baking powder and salt.
3 cups all-purpose flour, ½ cup granulated sugar, ¼ cup packed brown sugar, ½ teaspoon baking powder, ¼ teaspoon coarse kosher salt, 1 tablespoon cornstarch
Add butter and mix on low speed until the mixture is evenly moistened, 1-2 minutes. The mixture should resemble wet sand.
1 ½ sticks unsalted butter
Add egg, vanilla, and almond extracts and mix until just combined, about 30 seconds. The dough will be crumbly, but it should stick together when you squeeze it in your hand.
1 large egg, 2 teaspoons pure vanilla extract, ¼ teaspoon almond extract
Divide the dough into 2 equal parts. Knead it into a smooth balls. Place each dough ball between 2 parchment sheets and roll it out into 1/4-inch thick disc. (TIP: Place the dough with parchment paper on a silicone mat to roll it out, so it stays in place. And this adjustable rolling pin is perfect for rolling out even dough.)
At this point, you can move on to shaping and baking the cookies right away. Or you can put the dough into plastic bags and refrigerate for up to 3 days, or freeze for up to 3 months.