Beat sugars and butter with a mixer at medium speed until well blended (mixture will resemble damp sand). Add egg and egg white, 1 at a time, beating until well blended. Stir in vanilla.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, and salt; stir well with a whisk. Add flour mixture to sugar mixture; beat at low speed just until blended. Fold in 3 tablespoons butterscotch morsels.
Spread batter evenly into an 8-inch square baking pan coated with cooking spray. Sprinkle marshmallows, pecans, and remaining butterscotch morsels evenly over top; lightly press into batter. Bake at 350° for 30 to 31 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack. Cut into 16 bars.