Cream margarine and sugar in mixing bowl with paddle, about 2 to 3 minutes on low speed. Add flour, salt and egg. Mix on low speed just until smooth. Portion out two 10-ounce balls, wrap in plastic and refrigerate at least 30 minutes. Roll out one 10-ounce ball to fit into an 11-inch tart pan, prick with a fork and chill until ready to bake.
Bake in 375-degree oven until light brown, about 18 minutes.