Separate the white from the yolk of the eggs. Place the whites in a cool place.
In the bowl of the DMIX or BE8 put the soft butter with the powdered sugar. Add the egg yolks, flour, yeast and 15 cl of milk. Whisk to mix thoroughly. Set aside and preserve in a container.
Rinse the bowl of the DMIX or BE8 and dry it, then whip the egg whites to stiff peaks. Gently incorporate them into the previous preparation using a spatula.
Melt the chocolate with the remaining milk in a bain-marie.
Separate the dough in two containers by pouring 2/3 of plain dough into the first and 1/3 of plain dough into the second, mixing it with the melted chocolate.
Butter a crown mold. Pour half of the plain dough, the chocolate dough, then the rest of the plain dough. Give a few fork strokes in the dough to make the marbling.
Bake for 50 min. at 170/180°C. Let cool out of the oven before unmolding.