Preheat oven to 350 degrees and prep a muffin tin with liners.
In a large bowl mix the melted butter, granulated sugar, eggs, espresso and vanilla until blended. Highly recommend using a hand mixer or standing mixer if you have one.
In a smaller bowl, whisk the flour, cocoa powder, baking soda, baking powder, and salt
Alternate adding the flour mixture and buttermilk to the large bowl mixture. You should end by adding the last bit of the buttermilk.
Mix until combined and some streaks of flour remain
Stir in chocolate chips.
Fill muffin tins to 3/4 full. Sprinkle chocolate chips on top.
Bake for 15-18 minutes in center of oven. You will know its done when a toothpick comes out clean.
Let cool on a wire rack and enjoy!