Preheat the oven to 325 degrees F. Place a rack in the center of the oven. Line a 12-cup muffin tin with paper liners.
In a medium microwave-safe bowl, add the chopped chocolate and butter (you can do this on this stovetop also). Microwave for one minute. Stir. Continue microwaving at 30-second intervals, stirring after each time, until the chocolate and butter is melted and combined (don't overheat; microwave until just melted and smooth).
Stir in the sugar and vanilla until well-combined. Add the eggs one at a time, stirring or whisking vigorously after each addition.
Stir in the flour and salt.
Divide the batter evenly among the muffin cups. Bake for 17-20 minutes, until just baked through; don't overbake. The cupcakes won't be domed like a normal cakey cupcake - they'll be flatter and more dense like a delicious brownie.
Remove the cupcakes to a cooling rack to cool completely. Frost as desired and chill until ready to serve.
Notes
Nutrition Facts:the nutrition facts for this recipe were calculated based on the cupcakes, but do not include the frosting. Chocolate Chips: I use Ghirardelli chips (either bittersweet or semisweet) for the chocolate or chopped baking chocolate (usually found above the chocolate chips in the baking aisle - or the Trader Joe's pound plus bars work great, too).Chilled Cupcakes: keep in mind that these cupcakes taste best chilled, in my opinion. Of course, I wouldn't turn one away unchilled, but if you like the extra fudginess that comes when you chill brownies, plan ahead so they can have time in the refrigerator.Doubling: this recipe doubles great.