Preheat oven to 350°F. Line a 9x13 inch pan with foil and spray with cooking spray. Do not use a glass pan.

Mix butter and brown sugar with a stand or hand mixer until crumbly. Add eggs and vanilla; mix until smooth. Stir in salt, baking powder, then flour. Fold in M&Ms and sprinkles.

Spread the mixture evenly into the prepared pan. The dough will be wet, so use your sprayed hands to smooth it out. Top with additional M&Ms and sprinkles if desired.

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

Store in an airtight container for up to 4 days or freeze for up to a month.