Preheat the oven to 350 degrees F. Butter an 8-inch square baking pan.
In a large bowl, using an electric mixer, cream the butter and sugars until pale and fluffy. Beat in the egg and vanilla extract.
In a small bowl, combine the flour, baking powder and salt. Fold the dry ingredients into the wet ingredients and stir 1 cup of in your preferred mix-ins. Transfer to the prepared baking pan and spread into an even layer and press the remaining mix-ins on top.
Bake for 22-25 minutes until golden brown. Let the pan cool on a wire rack before cutting into 16 equal bars.