Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease lightly.
In a large bowl, whisk flour, espresso powder, baking powder, baking soda, salt, and sugar.
In another bowl, whisk melted butter, eggs, cooled brewed coffee, milk, and vanilla extract until smooth.
Pour wet mixture into dry ingredients. Gently fold until just combined. Do not overmix.
Fold in chocolate chips or nuts if desired.
Divide batter evenly, filling each cup about ¾ full.
Bake for 18–22 minutes or until a toothpick inserted in the center comes out clean.
Let muffins cool in pan for 5 minutes, then transfer to a wire rack.