Melt the butter in a wide-bottomed pan. Set aside to cool.
Pre-heat your oven to 175°C and prepare an 8×8 inch baking pan, lining and greasing it with parchment paper.
Sift the flour into a bowl. Whisk in the baking powder, cornflour and salt and mix well. Set aside.
Once the melted butter has cooled, beat the sugars and melted butter together until well-mixed and fluffy. Beat in the egg and vanilla extract until the mixture is light and evenly mixed.
Gently fold in the dry ingredients until no visible streaks of flour remain.
Now stir in ¾ cup of the chopped Oreo cookies, reserving the rest to sprinkle on top.
Pour the Oreo blondie batter into your prepared tin and smoothen the surface with a rubber spatula/ offset spatula. Add the remaining Oreo cookie pieces on top, gently pressing down.
Pop the blondies into bake for 16 – 22 minutes (baking times vary from oven to oven). Remove them from the oven when they look *almost done*. The edges will have browned, but the centre will look a tad underdone (with a slight wobble). Be careful not to over-bake in the oven, as if you do, your blondies will be less moist, instead becoming hard. No one wants over-baked blondies!
Let the blondies completely cool down on a wire rack before slicing them generously and serving. Preferably with a cold glass of milk or a hot cup of coffee to dunk in!