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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.
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2
Heat milk in a 2-quart heavy saucepan over moderate heat until hot but not boiling.
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3
Add one third of hot milk to yolk mixture in a slow stream, whisking constantly.
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4
Pour remainder of milk into yolk mixture, whisking, then transfer to saucepan.
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5
Simmer, whisking constantly, until very thick, 1 1/2 to 2 minutes.
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6
Remove from heat and add chocolate, liqueur, butter, and espresso powder.
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7
Let stand until chocolate is melted, about 1 minute, then whisk until smooth.
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8
Force custard through a fine sieve into a bowl.
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9
Cover surface of custard with wax paper and chill until cold, at least 4 hours.
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10
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).
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11
Turn out dough onto a sheet of plastic wrap and gather into a ball.
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12
Flatten dough into a disk and chill, wrapped in plastic wrap, until firm, at least 30 minutes and up to 8 hours (overnight).
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13
Preheat oven to 350F.
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14
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.
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15
Arrange molds 1 inch apart in a shallow baking pan and chill until dough is firm again, about 30 minutes.
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16
Bake in middle of oven, rotating pan halfway through baking, until pale golden, 12 to 15 minutes total.
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17
Transfer molds to a rack to cool, then carefully remove shells from molds.
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18
Make more tartlet shells in same manner if desired.
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19
Fill pastry bag with custard and pipe decoratively into shells.
