Preheat the oven to 350 degrees F. Line a muffin tin with paper or foil liners and set aside.
Put the butter and dark chocolate in a heat-proof bowl over a saucepan of simmering water (make sure that the bottom of the bowl isn't touching the water). Stir the butter and chocolate together until they are completely melted and combined. Whisk in the brown sugar and vanilla extract. Remove from the heat, set aside, and allow it to cool slightly.
After the chocolate mixture has cooled just a bit, whisk in the eggs one at a time, making sure that they are each thoroughly combined before adding another. Whisk in the flour, salt, and baking powder. Fold in the dark chocolate chips.
Divide the batter among the prepared muffin tins. Bake for 20-25 minutes, or until a toothpick, when inserted into the center, comes out clean. Allow the cupcakes to cool in the pan for 10 minutes before removing them to a wire rack to finish cooling completely.
Add the softened butter to the bowl of a stand mixer fitted with the paddle attachment. Beat it on medium for about 1 minute.
Add another 2 cups of the powdered sugar. Beat on low speed, and add the additional 1 cups of sugar as needed to thicken the frosting. Use the milk/heavy cream to thin things out as needed.
Notes: The buttercream recipe will very likely yield more than you need for 12 cupcakes.