1. beat the egg whites into a firm foam. 2. Mix the egg yolks with sugar and vanilla sugar. gradually add milk, oil, and flour mixed with baking powder. gently fold in the egg white foam. 3. Pour half of the mixture into a baking tray lined with baking paper (approx. dimensions 20x30 cm). 4. Add cocoa and a little milk to the remaining mixture. 5. Pour over the white mixture and gently run a fork through the mixture to create a marble pattern. 6. Distribute strawberries evenly over the mixture. 7. Bake at 170°c for about half an hour, or longer if needed. 8. Remove the cake from the oven and let it cool on a wire rack. 9. Sprinkle with powdered sugar and cut into squares.