In a medium mixing bowl, beat butter and sugar until very fluffy.
Add the vanilla sugar or extract, salt, and lemon zest and mix well.
Add the eggs one at a time, beating the dough for about ½ minute each time.
Add your sourdough starter and mix just until incorporated.
Mix flour and baking powder and gently fold them into the batter.
Preheat your oven to 350˚F.
Thoroughly grease your cake mold with extra butter. This is very important! Then dust it with flour, shaking out excess flour. Fill the cake mold with the batter. Depending on the size of your cake mold you might have extra batter. I just put it in a muffin pan or individual ramekins.
Bake for about 45 minutes. Since every oven is different, check after 30 minutes. The cake is done when an inserted wooden toothpick comes out clean. Let the cake sit in the mold for 10 minutes before removing it.