In a large bowl, mix sugar, yeast, salt, cinnamon and 2 cups flour. In a small saucepan, heat milk and butter to 120° -130°. Add to dry ingredients; beat on medium speed 2 minutes. Add eggs; beat on high 2 minutes. Stir in enough remaining flour to form a stiff dough (dough will be sticky). Turn dough onto a floured surface; knead until smooth and elastic, 6-8 minutes. Place in a greased bowl, turning once to grease the top. Cover and let rise in a warm place until doubled, about 1 hour. Meanwhile, to make streusel, in a large bowl, beat butter, flour, sugar and vanilla until combined. Divide in half. Mix chocolate into 1 half; set aside. Punch down dough. Divide dough into 12 portions; form each into a 3-in. oval. Place 2 in. apart on parchment-lined baking sheets. In a small bowl, whisk egg with milk; brush over dough. Divide plain streusel into 6 portions; roll each into a 3-in. circle. Place over half the rolls. Repeat with chocolate streusel and remaining rolls. Using a sharp knife, cut through streusel on top of rolls to resemble a clamshell. Cover with kitchen towels; let rise in a warm place until almost doubled, about 30 minutes. Preheat oven to 375°. Brush rolls with remaining egg wash. Bake until tops are lightly browned, 15-20 minutes. Remove to a wire rack to cool completely.