Preheat your oven to 350°F (180ºC) and line two baking sheets with parchment paper or silicone mats.
In a stand mixer, beat together softened butter, peanut butter, brown sugar, and granulated sugar until the mixture is fluffy and creamy. Add eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
1 cup peanut butter, 1 cup unsalted butter, 1 cup light brown sugar, ⅔ cup sugar, 2 large eggs, 1 teaspoon pure vanilla extract
In a separate bowl, whisk together flour, baking powder, baking soda, cornstarch, and salt. Slowly add the dry ingredients to the wet mixture, mixing until just combined—avoid overmixing.
2 ½ cups flour, 1 teaspoon baking powder, 1 teaspoon baking soda, 2 tablespoons cornstarch, ½ teaspoon salt
Scoop and roll the dough into balls, then place them on the prepared baking sheets. Flatten each ball gently to form thick discs.
Bake for 12-15 minutes, until the edges are lightly golden brown. Allow the cookies to cool completely on the baking sheets.