In a small bowl, stir together 1/2 cup sugar, 1/3 cup flour, butter and 1 1/2 teaspoons cinnamon. Mix together with fork and sprinkle over unbaked muffins. Bake in preheated oven for 20 to 25 minutes, until a toothpick inserted into center of a muffin comes out clean.
Combine flour, sugar, 3/4 cup chocolate chips, cocoa powder, and baking soda in a large bowl. Whisk egg, yogurt, milk, vanilla, and vegetable oil in another bowl until smooth; pour into chocolate mixture and stir until batter is just blended. Fill prepared muffin cups 3/4 full and sprinkle with remaining 1/4 cup chocolate chips.