Beat butter with sugar. Continuing to beat, add the yolks, grated lemon zest, one at a time.
Sift flour with baking powder. Alternately add flour, poppy seeds and milk to the egg-butter mixture, knead each time until smooth.
Separately beat egg whites into foam. Gradually add them to the dough.
Preheat oven to 180'C. Put the dough into greased molds and bake for about 40 minutes. Cool the finished cupcakes, remove from the molds and sprinkle with powdered sugar.