Preheat oven to 350° F (180° C). Line a Perfect Portion Muffin Pan with paper cups and set aside.
Cream butter and sugar in a large mixing bowl. Beat in eggs, one at a time. Beat in lemon zest and vanilla extract. Whisk gluten-free flour, baking powder, and salt in a separate mixing bowl.
Alternately add dry ingredients with milk to creamed mixture. Spoon batter into muffin cups.
Bake for 18 minutes until light-golden, and a toothpick or knife tip comes out clean when inserted. Cool before icing.
Cream butter with icing sugar. Beat in egg, vanilla extract, and cream to make spreadable.