Preheat the oven to 350℉. Line muffin cups with paper liners.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, espresso powder, salt and 1 cup of the chocolate chips.
In a separate bowl, whisk together the sour cream, milk, oil, eggs and vanilla. Add this mixture to the dry ingredients, and stir the batter just until blended (do not over-mix).
Fill the prepared muffin cups with ¾-full and sprinkle with the remaining ½ cup of chocolate chips.
Bake for 15 to 17 minutes, until a toothpick inserted into the center comes out clean. Cool in the pan for about 10 minutes before removing to a rack to cool completely.