Best Ever Pumpernickel Loaf

Stir warm water, molasses, and yeast together in the bowl of a stand mixer. Let stand until the yeast softens and begins to form a creamy foam, about 5 minutes. Add canola oil.
Mix bread flour, whole-wheat flour, rye flour, cornmeal, cocoa powder, vital wheat gluten, and salt in a bowl. Add slowly to yeast mixture. Fit the mixer with the dough hook attachment; turn to medium speed and let dough mix for 15 minutes.
Turn dough onto a floured surface; knead by hand until no longer sticky, but smooth and elastic, adding more flour as needed, 3 to 5 minutes. Grease a large bowl.
Transfer dough to greased bowl, turning to coat dough on all sides. Cover bowl; let rise in a warm area until doubles in size, 30 minutes. Grease a loaf pan.
Punch dough down and transfer to greased loaf pan. Cover and let rise in a warm area for 1 hour.
Preheat oven to 350 degrees F (175 degrees C).
Bake in preheated oven until loaf sounds hollow when tapped, 40 to 50 minutes. Cool on a wire rack.