USING A GRATER, grate lemon peels, removing only the yellow part. Place the grated peel in a mixing bowl. Squeeze the lemon juice and set aside in a small bowl. Combine the butter, vanilla, sugar and salt in an electric mixer fitted with a paddle, if you have one, and beat together at medium speed until creamy. Add the egg, 1/2 cup lemon juice and grated rind and beat until incorporated. Reserve any extra lemon juice for another use. Decrease speed to slow and add the semolina and then the flour, beating until just incorporated. Turn the dough onto a marble or wooden work surface and divide it into 2 balls. Wrap in plastic wrap and place in the refrigerator to chill for 1 hour. When chilled, remove from the refrigerator and roll each ball into a log 2-inches in diameter. PREHEAT OVEN TO 325F. Working on a marble or wooden surface, unwrap the dough log and cut into rounds about 1/2-inch thick. Sprinkle each round with a little sugar and lightly flatten the rounds with a rolling pin. Place the rounds on an unbuttered cookie sheet and chill in the refrigerator until firm, about 10 minutes. Bake the cookies on the middle rack of an oven for about 10 minutes, or until the surface starts to crack slightly. Remove from the oven and let cool. The cookies will be chewy at first, but will harden as they cool.