Step 1: Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.

Step 2: In a small saucepan, melt the butter over low heat, then transfer it to a large mixing bowl.

Step 3: Add the sugar to the melted butter and whisk until well combined.

Step 4: Beat in the eggs one at a time, then stir in the vanilla extract.

Step 5: In another bowl, whisk together the flour, cocoa powder, espresso powder, baking powder, and salt.

Step 6: Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Step 7: Stir in the chocolate chips, then fold in the brewed espresso and milk until the batter is smooth.

Step 8: Divide the batter evenly among the prepared muffin cups, filling each about 2/3 full.

Step 9: Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Step 10: Let the cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.