Beat the butter, granulated sugar, vanilla extract and egg in a large bowl on medium speed until combined. Stir in the flour and salt, then cover and refrigerate until firm, about 1 hour.
Prepare the lemon filling by beating all of the ingredients together until will mixed. Cover and refrigerate.
Preheat the oven to 350F Spray 48 miniature muffin cups 1 3/4 x 1 inch, with cooking spray.
Shape the dough into 48 one-inch balls then place in the muffin cup. Press dough into the bottom and up the sides of the cups. Spoon slightly less than 1 tablespoon of the lemon filling into each cup.
Bake 18 to 20 minutes or until centers are puffed and the edges are light brown. Cool in the pan for 5 minutes. With the tip of a knife, lift the tarts from the muffin tins to a cooling rack. Cool completely, then dust with the icing sugar just before serving.