Preheat oven to 375 degrees. Spray and line a 9x9” pan with nonstick spray and parchment paper.
In the bowl of a stand mixer, cream the butter and sugars until creamy. Then add the egg, vanilla extract, and milk and beat again.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Slowly add the dry ingredients to the butter mixture until a soft dough forms.
Press the dough evenly in the bottom of the prepared pan. Bake for 14-15 minutes, ensuring you don’t over bake.
While bars are baking, make the icing by creaming together the butter, powdered sugar, cocoa powder, vanilla extract and milk.
Allow bars to cool, then frost with the chocolate buttercream icing.
Add whole Ferrero Rocher Pralines around the outside of the bars, and finish by sprinkling them with crushed hazelnuts around the sides. Cut into 16 bars and enjoy!