How to Make It
Make the sponge. In a mixer, whisk together yeast and milk. Add bread flour and whisk to combine. Cover with remaining bread flour and all-purpose flour, then sugar. Sponge—which means letting the yeast sit, rise, and ferment a bit—in 70°F for 20 to 30 minutes. The flour will crack on top.
Place the water and milk into a medium saucepan and bring to a simmer over medium heat.
Dust work surface with all-purpose flour and roll half of brioche dough to 16-inch x 16-inch rectangle. Egg wash the entire surface.