Heat oven to 375°F. Line a 12-cup muffin tin with paper liners.
Whisk warm milk and espresso powder until dissolved. Set aside to cool slightly.
In a large bowl, combine flour, sugar, cocoa, baking powder, baking soda, salt, and cinnamon.
In a medium bowl, whisk espresso milk with oil, beaten egg, and vanilla until smooth.
Pour wet ingredients into the dry mix. Stir gently just until no dry spots remain.
Fold in chocolate chips. Do not overmix.
Divide batter evenly among liners. Sprinkle tops with coarse sugar if using.
Bake 16–18 minutes, until set and lightly springy. Cool 5 minutes in pan, then transfer to a rack.