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

Cream butter and brown sugar in a large bowl until fluffy, about 1-2 minutes. Add eggs, baking powder, vanilla, and salt; mix until smooth. Gradually add flour and mix well. Fold in Valentine M&Ms.

Press batter into the prepared pan and bake until the edges are very light golden, approximately 20-25 minutes. Let cool completely before slicing into bars.

Store in an airtight container for up to 3 days or freeze for up to 3 months.