Preheat oven to 350. Line 13" x 9" pan with foil sprayed with cooking spray; set aside. To prepare crust, combine one cup flour, sugar, and margarine. Mix until crumbly. Press into bottom of prepared pan. Bake for 15 minutes. To prepare brownie, combine remaining flour, sweetened condensed milk, cocoa powder, egg, vanilla extract, baking powder, chocolate pieces, and walnuts. Mix well. Spread mixture onto baked layer. Bake for 20 minutes, or until set. Cool. Use foil to lift brownies out of pan. Cut with heart shaped cookie cutters. Decorated as desired using decorating gel.