Preheat oven to 350°F. Lightly coat an 8x8-inch square baking pan with butter or nonstick spray, or line with parchment paper.
In the bowl of a stand mixer with a paddle attachment (or using a bowl with an electric mixer), beat together powdered sugar, butter, and vanilla until light and creamy.
Beat in flour. It will be crumbly at first, but keep beating until it comes together and cleans the sides of the bowl.
Press dough into an even layer in the prepared baking pan. Use a knife or bench scraper to cut into rectangles, squares, or diamonds. Dock the dough with a fork.
Bake for 20-30 minutes, until lightly golden. Let cool 5 minutes, then cut again along the same lines. Let cool completely.
Notes
Weight measurements are recommended.
If using granulated sugar, the cup measurement is ½ cup.
If using unsalted butter, add ¼ teaspoon salt to the dough.