HEAT oven to 350° F. Grease and lightly flour 9-inch-round cake pan.
COMBINE flour, cocoa, baking powder and salt in small bowl. Combine sugar, butter and water in medium bowl. Stir in egg and vanilla extract. Stir flour mixture into sugar mixture. Spread into prepared cake pan.
BAKE for 18 to 20 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack for 10 minutes. Run knife around edge of brownie; invert onto serving plate. Immediately sprinkle semi-sweet morsels over hot brownie crust. Let stand for 5 minutes or until morsels are shiny. Gently spread chocolate over brownie crust.
SPRINKLE chopped candies and/or marshmallows in single layer over pizza. Serve warm or at room temperature; cut into wedges.