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Transfer one dough ball to a greased bowl. In a separate bowl, combine the cocoa powder and milk. Add this chocolate mixture to the second ball of dough. Knead thoroughly to ensure that the chocolate flavor is well integrated.
After kneading both doughs, place them in greased bowls, cover them with plastic wrap or a damp cloth, and let them rest for about 1 hour, or until they have doubled in size.
Step 3: Roll and Shape the Dough
Once risen, you can start the shaping process. On a lightly oiled surface, roll out the vanilla dough into an approximately 8x12 inch (20x30cm) rectangle. Repeat this process with the chocolate dough, rolling it out to the same dimensions.