Step 1

Place a rack in the center of oven and preheat oven to 375 degrees F. Lightly coat a standard 12-cup muffin pan with nonstick spray or fit with paper liners.

Step 2

Place a medium heat-proof bowl over a pan of simmering water. Add butter and half of the chocolate. Heat gently, stirring often, until chocolate is almost completely melted. Remove from heat and continue stirring, allowing the residual heat to melt the chocolate the rest of the way. Let the mixture cool for 5 minutes.

Step 3

Meanwhile, in a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt. Set aside.

Step 4

To the bowl with the cooled chocolate, whisk in the coffee, yogurt, egg, vanilla extract until smoothly combined. Pour the wet mixture into the bowl with the dry ingredients. With a rubber spatula, stir to combine. Stop as soon as the flour disappears.

Step 5

Stir in the remaining chopped chocolate. Divide the batter evenly between the 12 muffin cups.

Step 6

Bake the muffins for 16-20 minutes, until a toothpick inserted into center of a muffin comes out clean without any wet batter clinging to it. Place the pan on a cooling rack and cool in the pan for 5 minutes, then carefully unmold the muffins and transfer to the rack to finish cooling completely. Serve warm or at room temperature.