Melt 100 g butter, remove from heat and add 6 tbsp. milk. Leave to cool at lukewarm temperature. Stir yeast and 1 teaspoon sugar until liquid. 250 g flour,.
Knead 1 level teaspoon salt, yeast, whole eggs and the butter mixture with the dough hooks of the mixer to a smooth dough. Cover and leave to rise in a warm place for about 45 minutes.
Cut the bacon into strips. Peel and chop onion. Fry bacon and onion in hot oil until golden brown. Let cool off. Grease 8 wells of a muffin tray (with 12 wells) and dust with flour.
Set aside about 2 tbsp. of the bacon mixture. Knead the rest into the dough. Form into 8 balls of the same size and place in the muffin troughs. Cover and let rise for about 15 minutes.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Whisk egg yolk and 2 tablespoons of milk. Spread the dough with it. Sprinkle with rest of bacon mixture. Bake in a hot oven for about 20 minutes.