Preheat oven to 400 degrees.
In one bowl, whisk together the milk, butter, espresso, vanilla and egg.
In another bowl, whisk together the flour, sugar, baking powder, salt and chocolate chips.
Pour wet ingredients into bowl with dry ingredients and gently mix until batter is just combined.
Evenly fill 12 muffin cups with batter and bake 16-18 minutes, or until tops of muffins are just set. Do not over bake.