Preheat oven to 350°F. Line a 9x13 inch pan with foil and spray with cooking spray.

Cream together the butter and brown sugar using a mixer until crumbly. Add eggs and vanilla and mix until smooth. Stir in salt, baking powder, and flour. Fold in M&Ms and sprinkles.

Spread the batter evenly in the prepared pan, smoothing the surface with lightly sprayed hands if necessary. Top with extra M&Ms and sprinkles.

Bake for 22-25 minutes until golden around the edges. Let cool before slicing into bars.

Store in an airtight container for up to 4 days or freeze for longer storage.