1.Preheat oven to moderate, 180°C. Line 2 x 12-hole mini muffin pans with paper cases.
2.In a bowl, using an electric mixer, beat butter and sugar together until light and fluffy. Add egg and vanilla, beating well.
3.Fold in flour alternately with milk. Lightly stir in rhubarb and lemon zest. Spoon mixture evenly into patty cases.
4.Bake for 15-18 minutes. Cool in pans for 5 minutes then transfer to a wire rack to cool completely.