Combine the yeast, water, and milk in the bowl of an electric stand mixer and using a wooden spoon, stir until the yeast is dissolved. Add the flour and salt, and fit the mixer with the dough hook. Toss a kitchen towel over the mixer, covering the bowl as completely as you can — this will save you and the kitchen from being showered in flour.
Turn the mixer on and off in a few short pulses, just to dampen the flour, then remove the towel, increase the mixer speed to medium-low, and mix just until the flour is moistened, about 1-2 minutes. At this point, you will have a fairly dry, shaggy mass.
Scrape down the sides and bottom of the bowl with a rubber spatula, set the mixer to low, and add the eggs, followed by the sugar and the orange zest, if using. Increase the mixer speed to medium and beat until the dough forms a ball, about 3 minutes.
Reduce the speed to low and add the butter in 2 tablespoon-sized chunks, beating until each piece is almost incorporated before adding the next. (You will have a dough that is very soft, almost like a batter.) Increase the speed to medium-high and continue to beat until the dough pulls away from the sides of the bowl, about 10 minutes.
Lightly butter another bowl and transfer the dough to the bowl. Cover with plastic wrap and leave at room temperature until nearly doubled in size, about 40-60 minutes, depending on how warm the room is.
Deflate the dough by lifting it up and around the edges and letting it fall with a slap into the bowl. Cover the bowl with plastic wrap and put it in the refrigerator. Slap the dough down in the bowl every 30 minutes until it stops rising, about 2 hours, then leave the covered dough in the refrigerator to chill overnight.