Preheat oven to 325°F. Line 24 (2-1/2 inch) muffin cups with paper baking cups.
For Cupcakes: Beat butter and cream cheese in a large mixing bowl until creamy. Add sugar and beat 2 to 3 minutes until light and fluffy. Beat in eggs, one at a time, until well blended, then corn syrup and vanilla.
Sift or whisk flour, corn starch, baking powder and salt together. Gradually add dry ingredients to butter mixture, mixing after each addition until combined. Fold in lemon juice and zest. Portion batter evenly into prepared muffin cups.
Bake 30 to 35 minutes or until edges are light golden and centers of muffins are set. Cool 10 minutes in pans, remove from pans to finish cooling. Frost with Lemon Buttercream Frosting.
For Lemon Buttercream Frosting: Beat butter with corn syrup, lemon juice and vanilla. Gradually add powdered sugar and beat well until fluffy. If needed add milk, 1 teaspoon at a time, to reach fluffy spreadable consistency.