Preheat the oven to 350F and lightly grease two 12-cup muffin tins.
In a large bowl, cream together butter and sugar until light. Beat in eggs one at a time, followed by sour cream and vanilla extract.
In a small bowl, whisk together flour, baking soda and salt. Add half of flour mixture to the butter mixture, followed by the sour cream and vanilla extract, followed by the addition of the rest of the flour mix. Stir well between each addition and mix until no streaks of flour remain.
Stir the cocoa powder and the dark chocolate into the boiling water (easiest in a large measuring cup). Pour chocolate water into the rest of the batter and stir until uniform.
Evenly distribute batter into prepared baking cups. Bake for 13-15 minutes, until a tester comes out clean and the cakes spring back when lightly pressed. (It’s fine if you can’t fit both trays into the oven at the same time, just wait until one batch finishes before putting in the second pan)
Turn cupcakes out onto a wire rack to cool completely before frosting and filling.
ASSEMBLY...
Take a cooled cupcake and, using a small pairing knife, cut a cone of cake (1-inch across by 1-inch deep) out of the top. Trim off the pointy end of the cone, leaving a flat circle of cake. Set aside and repeat this process for all the cupcakes.
Take the cream filling and squeeze a tablespoon or so into each cupcake cavity, filling the hole up to the top with filling. Top off with a flat circle of cake to plug the hole and keep the filling in place.
Using a butter knife or a small offset spatula, frost each cupcake with a layer of chocolate buttercream frosting. Place a dollop of icing in the center of the cupcake (on top of the cut out circle of cake) and spread from the center to the sides of the cupcake.
If you have leftover filling, transfer it to a fresh ziploc bag and cut a very small opening in one corner. Pipe a swirly line down the center of each cupcake.
Store in an airtight container until ready to serve. ~~
There are several ways to fill a cupcake with jam, cream or some other type of filling. The easiest involves cutting the cupcake in half and spreading it with the filling, treating the cupcake as though it were a tiny layer cake. It is effective, but doesn’t give you the classic “look” of filled cupcake. A slightly more sophisticated way is to put your filling into a pastry bag fitted with a metal or plastic tip, poke the tip into the center of the cake, and squeeze in a bit of filling. This method is quick, but tends to leave only a small amount of filling in place.
My favorite way - and, in my opinion, the best way - is the method I used for the Devil’s Food Cupcakes. It gives you the maximum amount of filling and the best overall look for the cupcake. All you need is a paring knife, or other small, sharp knife with a narrow blade, to begin. Insert the knife into a cooled cupcake at an angle, halfway between the center and the outside edge.
The exact angle isn’t crucial, but it should be around 45 deg., and the knife should go into the cake about 1″. The idea here is to cut out a circular cone of cake.
Once you have your cone of cake, remove it and slice off the pointy end of the cone, leaving just the circular base (which was the very top of the cupcake). The filling will be put in the hole once filled by the cake cone, and the circular piece will be used as a plug to seal the filling in.
Filling should be added using a pastry bag or (my favorite) a ziploc bag with the corner cut off. There is no need to use a pastry tip to fill the cupcakes; a medium-sized opening in either type of bag will make filling the cakes a breeze.
Once the filling is in place, just pop the flat circle of cake back into place and continue to frost as you would with any other cupcake. You can see what the finished cake, sliced in half to reveal the cream filling, looks like.
