Mocha Tartlets

Make custard:

Step 1

Beat together yolks, sugar, cornstarch, and a pinch of salt in a bowl with an electric mixer until thick and pale, about 1 minute. Heat milk in a 2-quart heavy saucepan over moderate heat until hot but not boiling. Add one third of hot milk to yolk mixture in a slow stream, whisking constantly. Pour remainder of milk into yolk mixture, whisking, then transfer to saucepan. Simmer, whisking constantly, until very thick, 1 1/2 to 2 minutes. Remove from heat and add chocolate, liqueur, butter, and espresso powder. Let stand until chocolate is melted, about 1 minute, then whisk until smooth. Force custard through a fine sieve into a bowl. Cover surface of custard with wax paper and chill until cold, at least 4 hours.

Make tartlet shells while custard chills:

Step 2

Pulse together flour, sugar, salt, butter, egg, and vanilla in a food processor until dough is smooth and begins to form a ball (it will be soft, like cookie dough). Turn out dough onto a sheet of plastic wrap and gather into a ball. Flatten dough into a disk and chill, wrapped in plastic wrap, until firm, at least 30 minutes and up to 8 hours (overnight).

Step 3

Preheat oven to 350°F.

Step 4

Break off 1-teaspoon pieces of dough and press evenly into molds with floured fingers to form shells about 1/16 inch thick, pinching off excess from rim and making bottoms slightly thinner than sides. Arrange molds 1 inch apart in a shallow baking pan and chill until dough is firm again, about 30 minutes.

Step 5

Bake in middle of oven, rotating pan halfway through baking, until pale golden, 12 to 15 minutes total. Transfer molds to a rack to cool, then carefully remove shells from molds. Make more tartlet shells in same manner if desired.

Assemble tartlets:

Step 6

Fill pastry bag with custard and pipe decoratively into shells.