Separate the eggs. Beat the yolks, powdered sugar, flour and grated lemon zest in a food processor until very foamy. Beat egg whites until stiff, gradually adding sugar. Fold into the yolk mixture.
Carefully stir in warmed butter. Preheat oven to 170°C. Grease the cake pan and dust with flour. Pour in the dough and bake for about 30-45 minutes.
Check if the cake is done with the help of a cake tester. Turn out onto a wire rack and serve the sponge cake sprinkled with powdered sugar.