Mix flour, sugar and salt in a large bowl. Spread butter in flakes on top. Dissolve yeast in 400 ml of lukewarm water. Pour on the flour mixture and knead to a smooth dough with the dough hooks of the mixer. Cover and leave to rise in a warm place for about 45 minutes.
Knead the dough again well and quarter it. Form each portion into a strand (approx. 20 cm) on a floured work surface and cut into approx. 10 pieces. Form dough pieces, except for 1, into rolls with floured hands and put them together on a baking tray lined with baking paper to form a grape.
Form 3 leaves and 1 stem from the remaining piece of dough and place them on the bun. Cover and leave to rise in a warm place for about 30 minutes.
Whisk the egg yolk and 3-4 tablespoons of water. Spread the grape with it. Sprinkle leaves and stem with sesame seeds. Bake the buns in a preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for 30-35 minutes and then let them cool down on a cake rack.