Make streusel: In a small bowl, whisk together the flour and sugar. Drizzle in the melted butter and mix with your fingertips until the mixture lumps into unevenly sized crumbs. Refrigerate until ready to use.
Combine the chocolate, sugar and cinnamon. Brush a 8 x 4 x 2 1/2 inch pan, or 5 cup loaf pan with melted butter. Roll the dough into a rectangle measuring 9 x 22 inches and brush off any excess flour. Brush the dough generously with the melted butter, leaving a 1/2 inch along the top edge of the dough. Sprinkle the chocolate mixture evenly over the dough, leaving the top 1/2 inch border free. Reserve 2 tablespoons of the mixture. Sprinkle the streusel over the chocolate, reserving about 3 tablespoons.
Brush the 1/2 inch border along the top lightly with water. Beginning at the bottom edge, roll the dough into a log and pinch along the seam to seal it well. Moisten the ends of the log with water and bring them together to form a ring. Pinch the ends to seal. Gently twist the circle of dough twice into a double figure 8 and place in the prepared loaf pan, keeping the joint and seam down. Brush the top with melted butter.
Preheat oven to 350 degrees F. Cover the pan with a towel and set in a warm draft free place to proof for about 1 1/4 hours, or until a finger gently pressed into the dough leaves an indentation. It will not quite double, but will increase more than half again in size when fully proofed. Brush the top with more melted butter and sprinkle with the reserved chocolate/sugar and streusel. Bake 45 to 50 minutes, or until the dough in the crevices appears baked. If the top browns too quickly, cover loosely with foil. Cool on a rack, in the pan, for 15 minutes. Slip a knife around the edge to loosen it and turn out onto the rack. Cool before slicing.