Preheat your oven to 350 F. Line a baking sheet with parchment paper and spray the paper with nonstick cooking spray. Set aside.
In a medium bowl, add 6 tablespoons of butter, 4 tablespoons of brown sugar, and 2 tablespoons of granulated sugar. Using an electric or stand mixer, beat until fluffy (about 2-3 minutes). Add egg and beat until also combined and fluffy.
Add the dry ingredients (oats, flour, baking powder, baking soda, and salt). Beat until well blended, about 1 minute.
Pour the mixture onto the prepared baking sheet and press down so it’s an even layer.
Bake until golden and crisp, about 18 minutes. Remove from oven and let the cookie portion cool completely as you work on the filling.