In a mixing bowl, combine the flour, sugar, and salt. Add the softened butter and work together with your fingertips or a pastry blender. Make a well in the center of the mixture (push the flour mixture to the sides of the bowl) and add the egg yolk, vanilla, and lemon juice. Mix the wet ingredients together with your fingertips and slowly incorporate the dry ingredients until the dough forms a ball and no longer adheres to your hands. Cover with waxed paper and refrigerate for at least 30 minutes. Preheat the oven to 400 degrees Butter and flour a 9-inch tart or pie pan. Roll out the chilled dough on a floured board. Place the dough into the pie or tart pan and bake for 7 to 10 minutes, until golden brown.