Preheat the oven to 375 degrees F. Prep the muffin tin with butter or oil.
Add the milk, water, and coffee to a saucepan and let simmer.
Add the cocoa powder to the saucepan and mix. Then, add the butter and ½ cup of chocolate chunks, mixing until melted.
Transfer the melted mixture to a bowl to cool.
In a separate bowl, mix the flour, baking powder, and salt. To the bowl with the melted mixture, add the brown sugar, granulated sugar, vegetable oil, eggs, and vanilla extract, and mix.
Slowly add the flour mixture to the bowl, mixing with a folding spatula. Do not over mix. Fold in the ⅓ cup chocolate chunks. Evenly distribute the batter into the muffin tin. Top each muffin with more chocolate chunks. Bake for 24 minutes (bake time may vary).
For the filling: In a saucepan over medium heat, mix the heavy cream, chocolate chunks, and salt. Let cool slightly.
Once the muffins are ready to be filled, use a piping bag of the filling to fill the centers of each muffin.
