Preheat your oven to 375°F (190°C). Line or grease a standard muffin pan to make 12 muffins.

In a large mixing bowl, combine the melted butter and granulated sugar. Mix until fully combined.

Add in the eggs and vanilla extract. Mix well until the mixture is smooth and creamy.

Stir in the whole milk and sour cream until fully incorporated.

In a separate bowl, whisk together the all-purpose flour, dark cocoa powder, baking powder, and salt. Ensure they are well mixed.

Gradually fold the dry ingredients into the wet mixture. Mix until just combined, being careful not to overmix.

Fold in the chocolate chips and chocolate chunks, ensuring they are evenly distributed throughout the batter.

Scoop the batter evenly into the prepared muffin pan, filling each cup about ¾ full.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

Allow the muffins to cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely.

In a small saucepan, heat the heavy whipping cream over medium-low heat until it just begins to simmer.

Remove from heat and add the chocolate chunks to the warm cream. Let it sit for about 2 minutes to melt.

Stir the mixture until it becomes smooth and glossy.

Once the muffins are completely cooled, use a small knife or a cupcake corer to create a small hole in the center of each muffin.

Fill a piping bag with the ganache and pipe it into the center of each muffin until filled.