PREHEAT oven to 375° F. Paper-line 18 muffin cups.
COMBINE flour, granulated sugar, brown sugar, baking powder and salt in large bowl. Stir in milk, egg, butter and lemon peel. Stir in 1 1/2 cups morsels and blueberries. Spoon into prepared muffin cups, filling almost full. Sprinkle with Streusel Topping.
BAKE for 22 to 25 minutes or until wooden pick inserted in center comes out clean. Cool in pans for 5 minutes; remove to wire racks to cool slightly.
PLACE remaining morsels in small, heavy-duty plastic bag. Microwave on MEDIUM-HIGH (70%) power for 30 seconds; knead. Microwave at additional 10- to 15-second intervals, kneading until smooth. Cut tiny corner from bag; squeeze to drizzle over muffins. Serve warm.
FOR STREUSEL TOPPING:
COMBINE granulated sugar, flour and cinnamon in small bowl. Cut in butter or margarine with pastry blender or two knives until mixture resembles coarse crumbs.