In a small, heat-safe bowl, place the chopped chocolate and butter. Place the bowl over a small pot of simmering water, making sure the bowl doesn’t touch the water.

Cook, stirring frequently, until the chocolate and butter are melted and smooth. Set aside to cool briefly. In a large bowl, place the flour, xanthan gum, cocoa powder, baking powder, baking soda, salt and sugar, and whisk to combine well.

Create a well in the center of the dry ingredients and add the melted chocolate and butter, then the sour cream, eggs and vanilla, mixing until just combined after each addition. The batter will be thick.

Fill the prepared wells of the muffin tin 2/3 of the way full. Bake until a toothpick inserted in the center of each cupcake comes out clean (about 20 minutes).

Remove from the oven and allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely. Repeat with the remaining batter.

Place all of the frosting ingredients in a small, heat-safe bowl and whisk to combine well. Place over a small pot of simmering water, making sure the bowl doesn’t touch the water and that there is enough water in the pot to simmer for at least 4 minutes.

Clip a candy/deep fry thermometer to the side of the bowl and cook until the mixture reaches 160°F (about 4 minutes). If it seems that the temperature of the mixture is rising very slowly, cover the bowl with the lid of the pot, as it sits over the simmering water.

Remove from the heat and transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment or a large bowl with a handheld mixer. Beat on medium-high heat until the mixture holds a very stiff peak (about 7 minutes).

Transfer the frosting to a large piping bag fitted with a large open piping tip (about 1/2-inch in diameter). Pipe a spiral about 2-inches high on the top of each cooled cupcake, and place it on a rimmed baking sheet. Place the cupcakes in the refrigerator to cool while you make the chocolate shell.

Place the chocolate shell ingredients in a small, but deep, heat-safe bowl, and place over a small pot of simmering water, making sure the bowl doesn’t touch the water. Cook, stirring frequently, until the chocolate and oil are melted and smooth. Remove from the heat and set aside to cool for about 10 minutes. The mixture should be thin.

Working quickly, dip each frosted cupcake straight down into the chocolate shell by holding the bottom of the cupcake and covering as much of the frosting with the shell as possible.

Lift the cupcake straight out of the melted chocolate shell and invert onto a wire rack.

Allow to stand at room temperature, or refrigerate, until set (about 20 minutes—less in the refrigerator).