Preheat the oven to 180 °C (top/bottom heat).
Mix the flour with the baking powder and sift. Add the sugar, vanilla sugar, lemon zest, eggs and melted butter or margarine. Using a hand mixer, beat first on low, then on high speed for about 2 minutes until smooth. Stir crème fraîche and poppy seeds into the dough.
For the crumble, knead 6 tbsp. flour with 100 g butter, 3 tbsp.
sugar and the vanilla sugar and crumble. Pour the batter into muffin cups and sprinkle the crumble on top. Place in the middle rack of the oven and bake the muffins for 15 minutes, let cool on a cooling rack for 10 minutes. Dust with powdered sugar. Tip: If you like, you can also put tangerines in the batter.
