Mix the sponge ingredients in the bowl of your mixer.
½ cup (62 grams or 2.2 ounces) bread flour, 1 tablespoon sugar, 2 teaspoon instant yeast, 1 cup warm water
Top with the dry ingredients for the dough (flour, sugar, milk powder, zests), cover with plastic wrap, and let the bowl sit at warm room temperature for 1 to 2 hours, until the sponge is starting to bubble through.
3 cup (375 grams or 13.2 ounces) bread flour, 4 tablespoons sugar, ¼ cup powdered milk, zest of 1 small orange, zest of 1 lemon
Mix at low speed, adding the butter and then the eggs and vanilla and then the salt.
4 tablespoons (57 grams or 2 ounces) unsalted butter, room temperature, 2 eggs, room temperature, 1 teaspoon vanilla extract, 1 teaspoon salt
When the dough comes together, knead in the mixer for 10 to 12 minutes. The final dough should be supple and soft and not wet or sticky.
Oil the bowl and let rise at warm room temperature until doubled in size, about 2 hours.
Punch down and divide into two equal pieces and shape into loaves in buttered 9 inch by 5 inch loaf pans or, per the original, into boules in two oiled 9 inch pie or cake pans. P
Proof at warm room temperature until doubled in size. For loaves, they will reach about an inch or so above the loaf pan.
Preheat the oven to 350°F and bake for 50 to 60 min until the internal temperature reaches 190°F. Check for burning after 30 minutes and cover loosely with foil if the tops are getting too brown too quickly.