With a mixer, in a food processor or by hand mix butter and sugar. Add the eggs, zest, vanilla sugar. Stir.

Add the yoghurt. Stir

Mix the flour, baking powder and salt. Add to the dough. Stir.

Here's the consistency of the dough we've got. Put the dough in the form. Preheat the oven to 180 C. Bake for approximately 35-45 minutes. To check the readiness with a wooden skewer or toothpick. If the stick comes out clean of dough, then the cake is ready. Still warm greased muffin impregnation. For its preparation put the sugar in a small pan. Add water (1:1 ratio). Bring to boil and simmer on very low heat until the sugar is completely dissolved and the syrup will become transparent and slightly darkened (about 10 minutes). Not to stir. Cool, pour into jar or bottle. Store in the refrigerator or at room temperature. Soaked the cake to cool slightly in the form. Then gently take out (the cake is delicate and could break apart) and cool completely on wire rack.