Preheat oven to 350 degrees.
Line a 9x9 pan with parchment paper. Spray parchment lightly with non-stick cooking spray. Set aside.
Brownie batter: In a small bowl, sift together flour, cocoa powder, salt and baking powder. Set aside.
In a medium bowl, cream butter and sugar together until fluffy. Add eggs, one at a time and vanilla and mix until well combined.
Slowly add the flour mixture and stir until just combined. Do not over mix. Pour the brownie batter into the prepared pan.
Top brownie batter with 16 Oreos, 4 across by 4 down, evenly spaced.
Blondie Batter: In a small bowl, sift together flour, baking powder and salt. Set aside.
In a medium bowl, combine melted butter and brown sugar until well blended. Whisk in the beaten egg and vanilla until combined. Add the flour mixture and and mix just until incorporated. Do not over mix. Fold in the chopped white chocolate.
Spread batter evenly over top of the Oreos.
Bake until skewer inserted into center comes out with a few moist pieces on it, about 25-27 minutes. Do not over bake. Remove from oven to wire rack to cool. Remove from pan by the parchment paper and cut into 2 inch squares.