Preheat oven to 350°. Grease 13" by 9" metal baking pan. Line pan with foil; grease foil.
Prepare Chocolate Base: In heavy 3-quart saucepan, melt margarine or butter with chocolates over low heat, stirring frequently. Remove saucepan from heat. With wooden spoon, stir in sugar and vanilla, then stir in eggs until well mixed. In small bowl, combine flour, baking powder and salt; stir into chocolate mixture just until blended.
Spread batter evenly in pan. Bake 25 to 30 minutes, until toothpick inserted 1" from edge comes out clean. Remove pan from oven. Cool completely in pan on wire rack.
Prepare Malted-Milk Topping: In small bowl, mix malted-milk powder, milk and vanilla until well mixed. Stir in margarine or butter, then confectioners' sugar, until blended. With small metal spatula, spread topping over cooled bars; top with chopped malted-milk-ball candies.
When topping is firm, transfer with foil to cutting board. Cut lengthwise into 4 strips, then cut each strip crosswise into 8 pieces.
Makes 32 bars.