Preheat the oven to 180°C (356°F). Line a 12-cup muffin tin with 12 muffin liners.

Brown 80 g unsalted butter in a small pan until it smells nutty and turns amber. Let the browned butter cool slightly.

Whisk 150 g caster sugar, the browned butter, 100 g plain yogurt, 60 g egg, and 1 teaspoon vanilla extract in a mixing bowl. Whisk until smooth.

Whisk 180 g all-purpose flour, 38 g unsweetened cocoa powder, 3/4 teaspoon baking soda, and 1/2 teaspoon salt in a second bowl. Add the dry mixture to the wet mixture.

Fold the batter gently with a rubber spatula until most dry streaks disappear. Do not overmix.

Stir 120 ml buttermilk and 40 ml espresso into the batter. Mix only until the thick batter is uniform.

Fold 150 g dark chocolate chunks into the batter. Reserve the 50 g dark chocolate chunks for topping.

Divide the batter evenly among the 12 prepared muffin liners. Sprinkle each muffin with the reserved dark chocolate chunks.

Bake the muffins at 180°C (356°F) for 18 to 20 minutes. Test a muffin with a skewer; it should come out mostly clean with a few moist crumbs.

Cool the muffins in the tin for 5 minutes. Transfer them to a cooling rack while still warm.

Warm 100 g chocolate ganache until fluid enough to pipe. Fill the warm muffins with ganache using a piping bag.