A hard chocolate shell surrounds layers of vanilla and chocolate ice cream enhanced with the flavor of malted milk powder. Each ball, made in two half-moon metal molds, can easily serve two people.
Chill molds in the freezer. Beat chocolate ice cream in the bowl of an electric mixer fitted with the paddle attachment until soft but still holding its shape, 1 to 2 minutes; work in batches if necessary. Wearing rubber gloves, remove molds one at a time from freezer; fill them with chocolate ice cream, pushing down on ice cream to prevent air pockets. Make the top even and smooth; return filled molds immediately to freezer. Let chocolate ice cream become firm, but not too hard to spoon out, about 1 hour. Remove molds from freezer one at a time; use a 1-ounce ice-cream scoop or round tablespoon to scoop out ice cream from center, leaving 1/2-inch border around edges. Return molds immediately to freezer.
Soften the vanilla ice cream as above. Add the malted milk powder, and stir just to combine. Remove molds from freezer, and fill each center with malted vanilla ice cream, smoothing top with an offset spatula. Return to freezer, and chill until firm, 1 hour more.
Working with two molds at a time, remove from freezer; dip in warm water for a few seconds. Use your finger to gently slide ice cream out of molds. Match two halves, flat sides together; wrap in plastic wrap. Gently press halves together; return to freezer. Repeat with remaining molds. Let balls harden, about 1 hour.
Melt chocolate in the top of a double boiler, or a heatproof bowl, over a pot of barely simmering water. When melted, remove from heat; stir chocolate occasionally to let it cool, 8 to 10 minutes. Place a wire rack over a rimmed baking sheet; set aside. Remove 1 malt ball at a time from freezer; dip in melted chocolate. Using a spoon or small offset spatula, very quickly turn to coat ball and lift out of chocolate. Place on wire rack, and let excess chocolate drip off (chocolate will adhere to wire rack if allowed to sit too long). Gently transfer malt-ball bombe to a waxed paper-lined tray; return to freezer before chocolate is completely set. Repeat process with remaining balls. If not serving immediately, wrap each ball in plastic wrap after it is completely frozen. Serve on plates with forks.