Proof the yeast. Warm the buttermilk to 105–110°F. Whisk in the yeast and a pinch of the sugar; let stand 10 minutes until foamy. If it doesn't foam, start over with fresh yeast.

Make the dough. In a large bowl, whisk the whole wheat flour, 1 cup all-purpose flour, remaining sugar, and salt. Add the yeast mixture, honey, and softened butter; stir into a shaggy dough. Add remaining all-purpose flour 1 tablespoon at a time until the dough pulls from the bowl but stays slightly tacky.

Knead. Knead on a lightly floured surface 10–12 minutes by hand (or 7–8 minutes on medium in a stand mixer) until smooth and elastic.

First rise. Place in an oiled bowl, turn to coat, cover, and rise in a warm (75–80°F) spot for 60–90 minutes, until doubled.

Shape. Punch down and divide in half. Pat each half into a 7×5-inch rectangle and roll up tightly from the long side; pinch seams closed. Place seam-side down on a parchment-lined baking sheet or in greased 8.5×4.5-inch loaf pans.

Top and second rise. Brush tops with water and press on the mixed grains. Cover loosely and rise 30–45 minutes, until puffy.

Bake. Preheat oven to 375°F (190°C). Slash 3–4 shallow cuts across each loaf. Bake 25–30 minutes, until deep golden and 190–200°F internally.

Cool and serve. Cool 15 minutes, slice, and serve warm with whipped butter or honey cinnamon butter.