This double chocolate chip muffin recipe uses both chocolate and butterscotch chips, but you can use all chocolate if you prefer.
Preheat oven to 350F.
Grease a muffin tin with butter or oil.
Whisk together flour, baking soda, sugar, and cocoa.
Beat together butter and sugar. Mix in egg, yogurt, milk, and vanilla until its incorporated. Add in flour mixture, and mix in till smooth. Fold in chocolate chips and butterscotch chips.
Fill prepared muffin tin 3/4 of the way. Sprinkle some chocolate chip and butterscotch chip on top of muffins. Bake for 15 to 20 minutes, or until an inserted toothpick comes out clean. Set to cool, once cool enough to touch turn muffins out and let cool completely on a wire rack.