Preheat Oven and Line Tin:

Preheat your oven to 350°F (175°C). Insert paper liners into a muffin tin.

Whisk Dry Ingredients Together:

In a large mixing bowl, combine flour, sugar, cocoa powder, baking powder, and salt with a whisk.

Mix Wet Ingredients Uniformly:

In another bowl, beat eggs, milk, oil, vanilla extract, and espresso together until uniform.

Fold Wet into Dry Gently:

Add the wet mixture to the dry ingredients and fold gently until barely combined. Avoid overworking the batter for a light texture.

Melt Chocolate and Mix Chips:

Melt the filling chocolate in a saucepan or microwave, then mix in the chocolate chips.

Fill Cups Half Full:

Spoon batter into each muffin cup until roughly half full.

Add Chocolate-Espresso Center:

Drop 1 teaspoon of the chocolate-espresso blend into the center of each portion.

Cover with Remaining Batter:

Cover with the remaining batter, filling each cup to about three-quarters full.

Bake Until Firm and Browned:

Bake for 18-20 minutes, until the tops are firm and lightly browned.

Cool Muffins on Rack:

Allow muffins to rest in the pan for several minutes, then move them to a wire rack to finish cooling.