Preheat oven to 350°F. Line a 9x13” pan with foil and spray with nonstick cooking spray.
Cream brown sugar and butter in the bowl of an electric mixer fitted with a paddle attachment. Add eggs and vanilla and beat until combined. Add baking powder, salt, pumpkin pie spice, and flour and mix. Stir in white chocolate chips.
Press into prepared pan (dough is sticky, spray your hands with cooking spray to avoid it sticking to your hands).
Bake for about 22-25 minutes. (Be careful not to overcook - the center will still be jiggly when you take these out of the oven and they will finish cooking and firming up as they cool.) Cool completely before slicing into squares.
Store in an airtight container for up to 4 days or freeze for up to 3 months.