Put butter, sugar, corn syrup and lemon peel in a small saucepan and stir over medium heat until melted. Stir in flour. Set aside to cool. Preheat oven to 375 F (190 C). Grease a large baking sheet; line with parchment or waxed paper. Using a round 1/4 teaspoon measuring spoon, spoon 12 tiny circles of mixture onto prepared baking sheet, spacing well apart. Bake 5 minutes or until lightly browned. Cool a few seconds. Lift rounds from baking sheet and place in minature tart pans to mold into a cup shape. When cool, remove to a wire rack. Repeat until mixture is all used.

To make filling, place butter, lemon peel and juice in a small bowl set over a pan of gently simmering water; stir until butter melts. Remove from heat. Stir in sour cream and powdered sugar. Beat until cool and thickened. Spoon or pipe filling into center of each cup.