Preheat your oven to 350°F (175°C) and line a muffin tin with parchment paper liners.
In a microwave-safe bowl, combine the chopped bittersweet chocolate and unsalted butter. Heat in the microwave in 30-second intervals, stirring in between until melted and smooth. Set aside to cool slightly.
In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt until well combined.
In another bowl, whisk together the brewed coffee, yogurt, egg, and vanilla extract until smooth. Add the melted chocolate mixture to the wet ingredients and mix until fully incorporated.
Pour the wet ingredients into the bowl with the dry ingredients. Gently fold the mixture together using a rubber spatula until just combined. Be careful not to overmix; it’s okay if a few streaks of flour remain.
Fold in the remaining chopped bittersweet chocolate, ensuring even distribution throughout the batter.
Spoon the batter into the prepared muffin tin, filling each liner about ¾ full. This allows room for the muffins to rise.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a cooling rack. Enjoy them warm or at room temperature!
