Grease a 10-inch tart pan with a removable bottom with butter, and coat lightly with flour. Refrigerate the pan until needed.

Prepare the tart dough: In the bowl of an electric mixer fitted with the paddle attachment, on medium speed, beat the butter, sugar, and zest until smooth, scraping down the sides of the bowl, if necessary. Add the egg, and mix until smooth. Scrape down the sides of the bowl. Remove the bowl from the mixer and fold in the flour with a wooden spoon. Wrap the dough in plastic wrap and chill for 2 hours.

On a lightly floured work surface, roll the dough out evenly so that it is 12 inches in diameter.

Transfer the dough carefully to the chilled tart pan by draping it over a well-floured rolling pin, lifting it and gently setting it in the pan. Mold the dough in the pan, making certain the edges are square. Chill for 1 hour. Prick the dough with a fork, and chill for 30 minutes. Meanwhile, preheat the oven to 350 degrees.

Weight down the dough with aluminum foil and beans or pie weights. Bake 10 minutes, or just until the sides of the tart shell begin to brown. Remove the shell from the oven, and set aside.

Prepare the filling: In a medium-size bowl, whisk together the eggs, yolks, sour cream, sugar and lemon zest. Whisk in the citrus juices. Pour the filling into the prebaked tart shell. Bake for 25 minutes, or until the filling is firm. Cool to room temperature; then chill in the refrigerator for at least 2 hours before serving.

Makes one 10-inch tart; serves 10