1) Start sponge by mixing starter ingredients in a medium bowl. Stir until it makes a smooth batter. Cover and let sit at room temperature for 60-90 minutes, until the sponge is very foamy.
2) To make dough, combine sugar, salt, powdered milk, and butter in large bowl or with a paddle attachment in a stand mixer. Combine until smooth.
3) Mix in eggs, one at a time, and extracts.
4) Mix in the sponge and flour and switch to the dough hook. Mix on medium with the dough hook until the dough is very supple and soft. If it is at all wet or sticky add more flour by the spoonful. Alternatively, you can knead by hand on a floured surface until the desired consistency is reached. This will take 10-12 minutes with a mixer or 15-16 minutes by hand.
5) Add dough ball to a lightly greased bowl, cover and let ferment until it doubles in size, about 2 hours.
6) Remove dough from bowl and divide into two pieces. Form each piece into a tight round ball (a boule). Lightly oil two 9-inch pie pans and place one boule in each pan, seam side down. Cover loosely and let rise for another 2-3 hours, until the dough doubles in size.
7) Brush loaves with egg wash.
8) Bake loaves at 350 degrees for 50-60 minutes until they are a deep brown and sound slightly hollow when thumped.
9) Remove from the oven and let cool on wire racks. Let cool for at least 45 minutes before slicing.