Preheat oven to 350°F (175°C).

Grease and flour a cake pan.

Beat eggs and sugar until pale and thick.

Add vanilla sugar or lemon rind and mix.

In a separate bowl, whisk together flour and baking powder.

Gradually add the flour mixture to the egg mixture, mixing until just combined.

Melt the margarine and let it cool slightly.

Gently fold the melted margarine into the batter.

Gradually add milk, mixing until smooth.

Pour the batter into the prepared cake pan.

Bake for approximately 45 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely.