Preheat the oven to 350 degrees F. Line a cupcake pan with liners; set aside.
Add the butter and chocolate to a heat-proof bowl set over a pot of simmering water. Stir until the butter and chocolate are melted. Stir in the brown sugar and vanilla. Remove from heat and allow to cool.
Once the mixture is slightly cooled, add the eggs, one at a time, beating well after each addition. Whisk in the flour, salt and baking powder until just combined.
Divide the batter among the prepared cupcake liners. Bake 22 to 24 minutes, or until a toothpick inserted into the center comes out clean. Remove the cupcakes from the oven and allow them to cool for 2 to 3 minutes in the pan, then transfer them to a cooking rack to finish cooling.
To make the buttercream frosting, add the butter to the clean bowl of a stand mixer fitted with the paddle attachment. Add the confectioners’ sugar and mix until light and fluffy, about 4 minutes. Add the marshmallow fluff, heavy cream and salt; beat for an additional minute.
Pipe the frosting onto the cupcakes and sprinkle with toasted coconut. Enjoy!