To make the dough: Using an electric mixer fitted with the paddle attachment, beat the butter until soft and very smooth. Add the confectioners' sugar, and beat until blended.
Add the flour and salt. With the mixer running at low speed, beat the dough until it becomes crumbly. Add the egg, and continue mixing until the dough just comes together (in order to get the characteristically sandy texture of this dough, it's important not to overwork it).
Turn out the dough onto a work surface, and gather into a ball. Cut in 1/2, and shape each 1/2 into a disk. Wrap both pieces of the dough in plastic. Refrigerate at least 1 hour.
Lightly butter the insides of 2 (8 by 3/4-inch) round tart rings. Place the rings on a baking sheet lined with parchment paper.
Working on a lightly floured work surface, roll 1 disk of the dough to a 1/8-inch thickness. Fit the dough into a tart ring, pressing lightly against the bottom and up the sides. Run your rolling pin against the rim of the circle, and cut off the excess. Repeat with second disk of dough and tart ring. Chill 30 minutes.
Heat oven to 350 degrees F with rack in center. Line each tart shell with a circle of parchment paper, and fill shells with pie weights or dried beans.
Bake crusts 15 minutes, remove the paper and beans, and bake just until the edges of the crusts are golden, 3 to 5 minutes. Transfer baking sheet to a rack to cool.
To make the filling: Grate the zest of each lemon using the small holes of a box grater, and set aside. With a small knife, cut off enough of the top and bottom of each lemon, exposing the flesh. Stand the lemons on end, and carefully cut away the white pith and a tiny bit of the flesh from the entire surface of each lemon; the juicy sections of lemon should now be completely exposed. Lay the lemons on their sides, and cut each lemon
crosswise into 1/2-inch-thick slices; remove seeds.
Place lemon slices, whole eggs, egg yolk, granulated sugar, and heavy cream in the jar of a blender, and puree until smooth. Strain the mixture into a bowl, and whisk in the reserved lemon zest.
Lower the oven temperature to 300 degrees F. Scatter the raspberries over the bottom of the 2 crusts. Pour the lemon filling into crusts. Bake tarts just until the filling has set in the centers, about 25 minutes; the filling should not shimmy when the edges of the tarts are tapped lightly. Transfer tarts to a wire rack to cool. Serve.
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