Toffee-Topped Bars

(Makes 24 bars)

2 cups firmly [acked brown sugar
2 cups all purpose flour
½ cup (1 stick) butter or margarine, softened
1 tsp baking powder
½ tsp salt
1 tsp vanilla extract
1 cup milk
1 egg
1 cup semisweet chocolate chips
½ cup chopped walnuts
¼ cup unsweetened flaked coconut (optional)