In the bowl of a stand mixer fitted with a dough hook, combine yeast and sugar with 1/4 cup warm water. Stir until yeast is dissolved, then let stand to proof for 5 minutes or until foamy.
Stir in the remaining 3/4 cup water, then add flour, cocoa powder, salt and oil. Mix on low speed until the mixture pulls away from the sides of the bowl and forms a soft, elastic ball. (If the mixture looks too dry, add more water, a tablespoon or two at a time, until the dough comes together. If it's too wet, add more flour.)
Turn the dough out onto a clean work surface. Shape into a ball, then transfer to a lightly oiled mixing bowl and cover with a clean, damp towel. Set aside in a warm, draft-free spot to rise for about 2 hours or until almost doubled in size.
Divide the dough into 4 evenly-sized pieces. Gently knead each piece 3-4 times, then shape into a ball. Transfer to a large baking sheet, cover with a damp towel and let rise for another hour.