Heat up the milk and butter in a small pan or in a bowl in the microwave until the butter has melted. Let it cool down until just warm.
Stir in the sugar, the vanilla extract and add the yeast. Mix the flours with the salt in a large bowl or in the bowl of a standing mixer.
Pour in the milk mixture, add the egg and the egg yolk and mix with the dough hook attachment or by hand, with a wooden spoon.
Keep mixing or kneading by hand on a floured surface (it will be very sticky though) for at least 10 minutes or until the dough becomes smooth, elastic and clears the sides of the bowl or stops sticking to your hands. Place it in a clean bowl, cover and let it double in bulk in a warm place - it will take an hour.
Butter a round tin or a flan dish, about 30cm in diameter. Melt the remaining butter.
Turn the dough out onto floured surface and divide into eight pieces, 85g each.
Flatten them, then spoon a teaspoon of jam in the middle of each one.
Pull up the edges and pinch them well together, make sure there is no potential for leaks.
Roll each ball around on the worktop to check if it’s sealed.
Brush each dough ball with a little butter; otherwise you’ll end up with one gigantic Buchtel. Space them evenly in the dish, cover with greased cling film and leave to puff up for 40 minutes.
Preheat the oven to 180C/350F/gas 4. When the Buchteln have risen, brush the tops with any remaining butter and bake for 20 minutes until golden brown.
Cool slightly in the dish, dust with icing sugar and serve warm - traditionally with vanilla sauce or custard.
