In a medium bowl, sir together flour, sugar, 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 the dough just until a ball forms. If necessary, cover dough with plastic wrap and chill in the refrigerator for 30 to 60 minutes or until dough is easy to handle.

Preheat oven to 375 degrees F. Divide dough into 36 pieces. Press dough pieces onto bottoms and up sides of 1-3/4-inch muffin cups. Bake in preheated oven for 8 to 10 minutes or until shells are firm. Cool in muffin cups for 5 minutes. Remove shells from muffin cups. Cool completely.

To fill shells, prepare Peppermint Cream Filling. Using a pastry bag fitted with an open star tip, pipe Peppermint Cream Filling into shells. Sprinkle filling with crushed peppermint candies. Makes 36.

In a large bowl, combine 1/4 cup butter, softened; one 7-ounce jar marshmallow creme; and 1/2 teaspoon peppermint extract. Beat with an electric mixer on medium speed until smooth. Gradually beat in 1-1/2 cups powdered sugar. Beat in 1 tablespoon milk. Beat in 1 cup powdered sugar. Using a wooden spoon, stir in 1 cup powdered sugar. If necessary, stir in enough additional milk, 1 teaspoon at a time, to make a filling of piping consistency.

Layer unfilled shells between pieces of waxed paper in an airtight container; cover. Store at room temperature for up to 3 days or freeze for up to 3 months. Thaw shells, if frozen. Fill as directed in Step 3.