1. Warm the milk. Crumble the fresh yeast into the milk.
2. In the bowl of a stand mixer, pour the flour, egg, salt, and sugar. Add the milk and knead the dough. Add the butter last. Continue kneading until the dough pulls away from the sides of the bowl (add a little flour if necessary).
3. Cover the bowl with a damp cloth and let it rise for 1 hour in a warm place.
4. Punch down the dough and shape the brioche in its mold, or into a braid.
5. (Optional) If the brioche is molded, cover the mold with a damp cloth and let it rise in the oven at 80°F for another 20 minutes, or 1.5 hours in a warm place (to prevent cracks in the brioche during baking).
6. Brush with milk. Bake in the oven at 100°F for 15 minutes, then at 160°F for 30 minutes.