Using a stand mixer fitted with the dough hook , mix together the milk and yeast on low speed for 5 to 10 seconds to dissolve the yeast. Add the all-purpose flour, bread flour, salt, sugar, and the 2 tablespoons soft butter and continue to mix on low speed for 3 to 4 minutes, or until the butter is fully incorporated and the dough is smooth. Remove the dough from the mixer bowl, place it on a tray, and cover it loosely with plastic wrap. This dough block is called the détrempe. Place the tray in the refrigerator for at least 6 hours or for up to 12 hours. The dough will firm up and the yeast will take some action and proof a bit.
