For Cupcake:

Preheat the oven to 350F/180 C 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. 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.

For Vanilla Cream Filling:

Whisk together the flour and milk and cook in a small saucepan over medium heat until thick. This will only take a few minutes. Stir continuously to prevent the mixture from clumping and do not bring all the way to a boil. When thickened (consistency will be that of a thin pudding or custard), strain with a mesh strainer into a small bowl, cover with plastic wrap and let cool completely to room temperature.When the milk mixture is cool, cream the butter (or shortening) and sugar together in a medium bowl until light. Add in the milk/flour mixture and the scraped vanilla bean seeds (or vanilla extract) and beat at high speed with an electric mixer for 7 minutes, until light and fluffy. Scrape into a pastry bag fitted with a plain tip, or a large ziplock bag with the corner cut off, and set aside until ready to fill your cupcakes.

For Chocolate Buttercream:

Combine all ingredients in a large bowl and beat, with an electric mixer, until frosting is creamy, 3-5 minutes. Add additional powdered sugar as needed to achieve a thick, but not stiff, frosting that is easy to spread. Add additional milk, if necessary, to thin the frosting if it gets too thick.

To Assemble:

(A photo how-to of the assembly method can be found here.) 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.