Preheat the oven to 325°F.
Line muffin pans with paper cups (or grease the tins). This recipe makes enough to fill 4 standard 6-cup trays.
Melt chocolates and butter together in microwave for 30-second increments, stirring in between, until just melted. This should take about 2-3 minutes of microwave time. Allow this to cool, about 5 minutes.
Whisk together sugars, egg, & vanilla until fluffy (about 3 minutes).
Fold in cooled chocolate mixture.
In a separate bowl, stir together flour, baking powder, & salt. Fold the flour mixture into the wet ingredients.
Fill the prepared muffin tins ¾-full with batter. Bake for 15-20 minutes, until beginning to crack on the surface and the edges are puffed. When done, moist crumbs should stick to an inserted toothpick.
Allow to cool for at least 5 minutes, then transfer to a metal cooling rack until completely cool.
Transfer to refrigerator, chill at least 1 hour to set the fudgy center.