Crumble the yeast into the lukewarm milk and let it stand briefly. Then combine with the flour, butter, salt and sugar in a bowl and knead into a soft, smooth dough.
Cover the dough and let it prove at room temperature for approx. 1½ hours, until doubled in volume.
Divide the dough into two equal portions, shape each into a strand about 60 cm long and braid them together. Place on a baking tray lined with baking paper and let rest for another approx. 30 minutes.
Preheat the oven to 200 °C / 390 °F (conventional top and bottom heat).
Whisk the egg yolk with 1 tbsp milk or cream and brush it evenly over the braid.
Bake in the lower half of the oven for approx. 35 minutes until golden brown. If it browns too quickly, cover loosely with foil.
Let cool on a wire rack and enjoy fresh.