Preheat the oven to 180°C (approximately 350°F). Line a 12-cup muffin tin with paper liners or brush with butter and chill until ready to use.
Pour the flour into a large bowl and make a well in the center.
In a bowl, mix the crumbled yeast with 1 teaspoon sugar and 5 tablespoons of lukewarm milk until smooth. Pour the yeast mixture into the well and mix together to until smooth. Cover and let rise in a warm place for 10-15 minutes.
Pour the remaining sugar with the milk, butter, egg and vanilla sugar into the bowl and knead well. Cover and let rise in a warm place for 30 minutes.
Add the rest of the ingredients and knead well.
Pour the batter into the muffin tin, cover and let rise in a warm place for 15 minutes.
Brush the muffins with the whisked egg yolk and cut a cross in the top of each. Bake until a toothpick inserted in the center of a muffin comes out clean, 20-25 minutes. Cool in the pan for 5 minutes then remove and serve warm.