Warm milk to roughly 40°C (104°F), dissolve 40g yeast in it, add 50g flour, and stir until smooth. Dust the top with flour, cover, and let rise in a warm place for 30–60 minutes until roughly doubled.
Melt 80g butter or margarine.
Add the remaining flour, 3 eggs, 80g powdered sugar, 1 tablespoon vanilla sugar, 1 pinch salt, and lemon peel to the risen starter. Process with a food processor or wooden spoon until a smooth dough forms and separates from the bowl edge. Cover and let rest for 20 minutes.
Divide the dough into three equal parts and roll each into a long strand.
Brush each strand with plum jam and roll up.
Braid the three strands together, cover, and let rise for 20–30 minutes until puffy.
Brush the braid with melted butter and bake at 175°C (347°F) for 20–25 minutes until golden brown.