In this dessert, buttery pastry shells enclose a silky dark chocolate ganache filling highlighted with fresh lemon.
6 lemon peel twists (optional)
FOR PASTRY: Combine flour, powdered sugar and salt in processor. Using on/off turns, cut in butter until mixture resembles coarse meal. Add egg yolk, cream and vanilla. Process until moist clumps form. Gather dough into ball. Divide dough in half. Flatten each half into disk. Wrap in plastic. Refrigerate until firm, about 1 hour. (Can be prepared 1 day ahead. Keep refrigerated. Let dough soften slightly at room temperature before rolling.)
Position rack in center of oven and preheat to 375°F. Roll out 1 dough disk on lightly floured surface to 1/8-inch thickness. Using plate as guide, cut out three 6-inch rounds. Transfer rounds to 4 1/2-inch-diameter tartlet pans with removable bottoms. Press dough into pans. Fold in any excess dough; press to form double-thick sides. Repeat rolling, cutting and forming with remaining dough disk, making 6 crusts total. Freeze crust until firm, about 5 minutes.
Place crusts on large baking sheet. Bake until light golden brown, about 20 minutes. Transfer to rack and cool crusts completely. Remove crusts from pans.
FOR FILLING: Bring 3/4 cup cream to simmer in heavy medium saucepan. Remove from heat. Add chocolate and whisk until smooth. Mix in 1 tablespoon lemon peel. Divide chocolate among cooled crusts. Refrigerate tartlets until filling is set, about 1 hour. (Can be made 1 day ahead. Cover and keep refrigerated.)
Combine 1/4 cup cream and 1/2 teaspoon lemon peel in large bowl. Beat until soft peaks form. Spoon whipped cream into pastry bag fitted with star tip. Pipe cream decoratively atop tartlets. Garnish with lemon twists, if desired.