1. Preheat the oven to 350°F (175°C).

2. Cut the rhubarb into small pieces and mix with the granulated sugar.

3. In a separate bowl cream the butter and sugar until light and fluffy.

4. Add the grated lemon peel one egg at a time and mix well.

5. In another bowl whisk together the flour vanilla sugar and baking powder.

6. Gently fold the dry ingredients into the butter mixture until a smooth batter forms.

7. Spread the batter evenly onto a parchment-lined long pan (10x14 inch or 25x35 cm).

8. Top with the rhubarb mixture.

9. Sprinkle a mixture of ground cardamom and granulated sugar over the rhubarb.

10. Dot the top with butter.

11. Bake in the lower part of the oven for approximately 35 minutes.

12. Let cool and cut into squares.