Preheat the oven to 325 degrees. Line 18 muffin tin cups with paper liners.
Sift the flour, baking soda and salt together and set aside.
In a small microwave-safe bowl, melt the chocolate on 50% power for 1-2 minutes. Stir and continue melting on 50% power for 30 second intervals until the chocolate is melted. Set aside.
In a large mixing bowl, cream the butter and sugar together until light and fluffy. Blend in the vanilla. Add the eggs and beat until the batter is creamy. Blend in the chocolate. Add one-third of the sifted dry ingredients and mix just until combined. Add 1/2 of the sour cream and mix. Add another third of the dry ingredients and mix together. Add the other half of the sour cream and mix. Finally add the remaining dry ingredients and beat until well combined.
Fill each muffin cup about 1/2 full. Bake for 10-14 minutes, until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter). The key to not drying out these cupcakes is to make sure and not overbake them! Remove the cupcakes to a wire rack and let cool completely.
To make the frosting, combine the cream, cocoa, sugar and salt in a medium saucepan. Gently heat and stir constantly until the sugar has dissolved and the cocoa has blended into the mixture. Remove from the heat and let cool to room temperature. Pour the mixture through a mesh strainer into a medium glass bowl and cover with plastic wrap. Put in the refrigerator until completely chilled.
With an electric mixer, whip the frosting until it is smooth and forms medium peaks and has a spreading consistency. It will be quite firm. Frost and garnish the cupcakes as desired. I use a large piping tip (Wilton 1M tip to be exact) to make the decorative swirly frosting.
Notes
Frosting: after several comments on the frosting recipe alerted me to the fact that it wasn’t whipping up for many of you, I tested it. And sadly, I had the same result, even though it had whipped up perfectly the first two or three times I made it a few months ago. I plan on continuing to test it to see what’s going wrong – my guess is it has something to do with overheating the cocoa/cream mixture in the first step but I’m not entirely sure. So until I can troubleshoot it more, I want to add a caveat on to this recipe and in the comments that this frosting is temperamental (but absolutely divine if it whips up to stiff peaks)!