In a medium bowl stir together flour, sugar, malted milk powder, and cocoa powder. Using a pastry blender, cut in butter until mixture is crumbly. In a small bowl combine egg yolk and the cold water. Gradually stir egg yolk mixture into flour mixture. Gently knead dough just until a ball forms. Cover and chill for 30 to 60 minutes or until dough is easy to handle.
Preheat oven to 375 degrees F. Shape dough into 24 balls. Press balls onto the bottoms and up the sides of 24 ungreased 1-3/4-inch muffin cups. Bake for 8 to 10 minutes or until pastry is firm. If the centers puff during baking, press with the back of a measuring teaspoon while tassies are still warm. Cool in muffin cups on a wire rack for 5 minutes. Remove tassies from muffin cups. Cool completely on wire rack.
Spoon Cream Filling into a decorating bag fitted with an open star tip. Pipe filling into tassies (you will have more filling than you need). Sprinkle with chopped malted milk balls.
In a medium mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add marshmallow creme and vanilla; beat on medium speed until smooth. Beat in 1 1/2 cups of the powdered sugar and 1 tablespoon milk. Gradually beat in the remaining 2 cups powdered sugar and enough additional milk to reach piping consistency.