Preheat oven to 350 degrees. Line a 9" x 13" pan with aluminum foil and spray with cooking spray.
Combine melted butter and brown sugar in a large mixing bowl.
Add eggs and vanilla and beat well until smooth.
Combine flour, baking powder, and salt in a small bowl. Add to butter mixture and combine well. Stir in sprinkles.
Spread into greased pan and top with additional sprinkles. Bake for 30 minutes or until brown and toothpick inserted comes out clean.
Cool before cutting.