In a small bowl, combine the lukewarm water (about 38–43°C) with the active dry yeast and 5 g of the brown sugar. Stir gently and let sit 5–8 minutes until foamy. If using instant yeast, stir it directly into the dry ingredients and skip this step (still keep water warm).

Mix dry ingredients

In the bowl of a stand mixer fitted with the dough hook (or a large mixing bowl), whisk together the all-purpose flour, whole wheat flour, rye flour, cocoa powder, espresso powder, remaining brown sugar, and salt until evenly combined.

Combine wet and dry

Make a well in the dry ingredients. Add the foamy yeast mixture (or warm water if using instant yeast), molasses, and vegetable oil. Mix on low speed until the dough comes together, about 2–3 minutes. If mixing by hand, use a wooden spoon until shaggy dough forms.

Knead

Increase mixer speed to medium-low and knead with the dough hook for 6–8 minutes until the dough is smooth, slightly tacky but not sticky. If kneading by hand, turn the dough out onto a lightly floured surface and knead 8–10 minutes. Add small sprinkles of all-purpose flour (1–2 teaspoons at a time) only if the dough is excessively sticky.

First rise

Lightly oil a large bowl and place the dough inside, turning to coat. Cover with plastic wrap or a damp towel and let rise in a warm draft-free spot until doubled in size, about 60–90 minutes depending on temperature.

Shape & Proof

Shape

Gently deflate the risen dough and turn it out onto a lightly floured surface. Shape into a round boule or an oblong loaf suitable for a loaf pan. If making a boule, tuck the edges underneath to create surface tension; if using a 9 x 5-inch loaf pan, lightly oil the pan and press the dough in to fit.

Second rise

Place shaped dough on a parchment-lined baking sheet dusted with cornmeal or in the prepared loaf pan. Cover loosely with lightly oiled plastic wrap or a towel and let proof until puffy and nearly doubled, about 45–60 minutes.

Bake & Finish

Preheat oven

About 30 minutes into the second rise, preheat the oven to 230°C (450°F) with a baking stone or an inverted baking sheet on the middle rack. Place a shallow metal pan on the lower rack for steam.

Score

When the dough is ready to bake, use a sharp lame or serrated knife to make one long shallow slash along the top of the loaf (or a few decorative slashes). This helps controlled expansion.

Bake with steam

Carefully slide the loaf (on parchment) onto the preheated stone or baking sheet. Pour the 60 ml of hot water into the shallow pan on the lower rack to create steam, and immediately close the oven door. Bake at 230°C (450°F) for 12 minutes to get good oven spring and crust formation.

Continue baking

After the first 12 minutes, reduce oven temperature to 200°C (400°F), remove the steam pan, and continue baking for 20–25 more minutes, or until the internal temperature of the loaf reaches 95–98°C (203–208°F) for a fully baked pumpernickel and the crust is dark brown.

Cool

Remove the loaf from the oven and transfer to a wire rack. Let cool completely at least 2 hours before slicing — pumpernickel continues to set as it cools and slicing too early will make crumb gummy.