Butter a springform pan (24-26cm) and preheat the oven to 160 degrees.
Mix the butter with the sugar until frothy. Stir in the egg yolks one after the other. Then stir in the poppy seeds. Beat the egg whites until stiff and fold into the batter. Pour the mixture into the springform pan and smooth it out. Bake the cake for 40 minutes (chopsticks test).