Whisk together the milk and yeast in the bowl of a stand mixer. Let sit 10 minutes to dissolve; you will see a few bubbles but the mixture won't be foamy. Whisk together the granulated sugar, eggs and 1 tablespoon salt in a small bowl.
Add the bread flour, whole-wheat flour, milk powder and egg mixture to the yeast mixture and mix with a wooden spoon until a shaggy dough forms. Fit the bowl in place on the stand mixer and fit the mixer with the dough hook. Mix on medium speed until the dough comes together but is still slightly tough, about 3 minutes. Increase the speed to high and add 20 tablespoons (2 1/2 sticks) butter a tablespoon at a time (the dough will look broken after each addition, but it will eventually come together) and beat until the dough is smooth, elastic and very tacky but pulls away from the sides of the bowl, about 12 minutes.
Cover the bowl with plastic wrap and let the dough rise in a warm, draft-free area until it has a buttery, yeasty smell and has doubled in volume, 35 to 50 minutes.
Meanwhile, whisk together the dark brown sugar, cinnamon and 1 teaspoon salt in a medium bowl until no lumps of sugar remain. Smash in the remaining 24 tablespoons (3 sticks) butter and stir until completely combined. Set aside until ready to assemble. Line two 18-by-13-inch sheet pans with parchment.
Gently deflate the dough by lifting it up around the edges and letting it fall back into the bowl, turning the bowl and repeating if needed. Transfer to a clean work surface lightly dusted with bread flour. Cut the dough in half. Roll one piece into a 12-by-18-inch rectangle, pushing the edges towards the center to make straight sides. Spread half of the butter mixture over the dough in an even layer, leaving a 1/4-inch border around the edges. Starting from a long end, tightly roll into a log, pressing the sides occasionally to keep them flush. Transfer to one of the prepared baking sheets and freeze for 20 minutes to firm up the dough. Repeat with the remaining dough and butter mixture.
Generously butter the bottom and sides of an 18-by-13-inch sheet pan. Cut one log of dough crosswise into 16 pieces and arrange them cut-side up in the pan, gently shaping each cinnamon roll to make them fit in the pan. Repeat with the remaining log, ending up with a 4-by-8 grid of rolls. Let rise, uncovered, in a warm, draft-free area until almost doubled in volume, 45 minutes to 1 hour.
Meanwhile, preheat the oven to 350 degrees F.
Bake the cinnamon rolls until golden brown on top and baked all the way through, 25 to 30 minutes. Remove to a rack to cool for about 10 minutes before glazing.
Meanwhile, whisk the confectioners' sugar, cream, vanilla and remaining 1/4 teaspoon salt in a small bowl until smooth and creamy, adding more cream if necessary to make a thick glaze. Drizzle the glaze over the cinnamon rolls and serve warm.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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