In a large bowl combine flour, sugar, 1 cup chocolate chips, cocoa powder and baking soda.
Whisk the egg with yogurt, milk, vanilla, and vegetable oil in another bowl until smooth; pour into the previous mixture and stir until batter is smooth.
Fill greased muffin cups 3/4 full and sprinkle with remaining 1/4 cup chocolate chips.
Bake until a toothpick inserted into the center comes out clean, about 20 minutes.
Cool 15 minutes in the pan.
Remove and cool completely on a wire rack before removing the muffin liners and serving.