Preheat oven to 350°F. Line two 9x9-inch square baking pans with parchment paper on the inside bottom and spray the entire inside (bottom and sides) with nonstick spray. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, eggs, vanilla extract, and peppermint extract until creamy, about 3-4 minutes.
¾ cup unsalted butter, 1½ cups granulated sugar, 3 large eggs, 2 teaspoons pure vanilla extract, ½ teaspoon pure peppermint extract
In a separate bowl, whisk the flour, baking powder, and salt together.
2¼ cups all-purpose flour, 2¼ teaspoons baking powder, ¾ teaspoon kosher salt
Pour half of the dry mixture into the wet mixture and beat just until mixed.
Pour all the buttermilk into the wet mixture and beat just until combined.
1 cup buttermilk
Add the remaining dry mixture to the wet mixture and beat until combined. Be careful not to overmix.
Divide the mixture evenly between the two baking pans (about 591 grams per pan).
Bake for 30 minutes, or until toothpick inserted in middle comes out clean.
Let cool completely before decorating.