Prepare Chocolate Pastry. Heat oven to 375 degrees F. Divide pastry in 24 balls. Evenly press each ball on bottom and sides of 24 ungreased 1-3/4-inch muffin cups, using floured fingers if necessary; set aside.
For filling, in small saucepan heat and stir chocolate and butter over medium-low heat until melted and smooth. Remove from heat. Stir in egg, sugar, liqueur, and vanilla. Spoon 1 scant tablespoon filling in each pastry shell.
Bake 12 to 15 minutes or until pastry is firm and filling is puffed. Cool in pans 10 minutes. Run sharp thin-blade knife around tassie edges; carefully remove from pans. Cool on wire rack. Pipe or dollop on a small amount of Chocolate Buttercream.
To store, refrigerate tassies in single layer in airtight container up to 3 days. Makes 24 tarts.
In food processor, combine flour, sugar, cocoa powder, and salt; pulse to combine. Add butter. Cover; process until crumbly. In small bowl, whisk together egg yolk and cold water. Add to processor; pulse until a dough ball forms (add water if dry). If needed, cover and refrigerate until easy to handle.
In a medium mixing bowl beat butter on medium-high for 30 seconds. Gradually beat in 1 cup powdered sugar and cocoa powder. Beat in milk. Gradually beat in 1 cup powdered sugar until piping consistency.