Preheat the oven to 375°F (conventional, no fan) and line a jumbo muffin tray with parchment paper or muffin liners, ensuring the liners rise slightly above the tray for height.

In a large bowl, sift together the flour, granulated sugar, brown sugar, baking powder, baking soda, and salt. Whisk to combine and eliminate any lumps.

Add the eggs, vegetable oil, and sour cream to the dry ingredients. Gently whisk until just combined, being careful not to overmix.

In a separate bowl, combine the cocoa powder, instant coffee, and boiling water, mixing until a smooth and shiny mixture forms.

Fold the cocoa mixture into the batter until just combined, avoiding overmixing.

Add the dark and milk chocolate chunks to the batter and gently fold them in.

Divide the batter evenly among the 6 jumbo muffin cups, filling them almost to the brim. If desired, reserve about ½ cup of batter to bake in a ramekin.

Top each muffin with additional chocolate chunks.

Bake in the preheated oven for approximately 27-30 minutes, or until a toothpick inserted into the muffins comes out with light crumbs (no wet batter).

Allow the muffins to cool for 20 minutes.

While the muffins are baking, prepare the ganache by heating the heavy cream until it just begins to boil. Pour it over the chocolate in a bowl, cover, and let it sit for 5 minutes before stirring until smooth.

Let the ganache cool at room temperature until it thickens.

Once the muffins have cooled slightly, transfer the ganache into a piping bag fitted with a round tip. Carefully lift each muffin, puncture the center with the piping tip, and fill generously with ganache, allowing some to ooze out on top.

Serve and enjoy! These muffins are best enjoyed warm when the ganache and chocolate chunks are still soft.