Preheat the oven to 350°F (175°C). Line standard cupcake pans with paper liners and set aside.

In a small bowl, whisk together the cocoa powder and hot water until smooth. Allow to cool to room temperature.

In a separate bowl, sift together the all-purpose flour, baking soda, baking powder, and salt. Whisk to combine and set aside.

In a medium saucepan over medium-low heat, combine the unsalted butter and granulated sugar. Stir continuously until the butter is fully melted and the mixture is smooth. Transfer the mixture to the bowl of a stand mixer fitted with the paddle attachment and beat on medium-low speed for 4 to 5 minutes until slightly cooled.

Add the eggs one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed. Add the vanilla extract and the cooled cocoa mixture, beating until fully incorporated.

With the mixer on low speed, add the dry ingredients in two additions, alternating with the sour cream. Mix until just combined; avoid overmixing.

Divide the batter evenly among the prepared cupcake liners, filling each approximately three-fourths full. Bake for 18 to 20 minutes, rotating the pans halfway through baking. A toothpick inserted into the center should come out clean. Allow the cupcakes to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Place the chopped bittersweet chocolate in a medium heatproof bowl. In a small saucepan, bring the heavy cream and corn syrup to a simmer over medium-high heat. Remove from heat and pour over the chocolate. Let stand for 2 minutes, then stir gently from the center outward until the mixture is smooth and glossy. Refrigerate, stirring every 5 minutes, until the ganache thickens to a pipeable consistency.

Transfer the ganache to a piping bag fitted with a round decorating tip. Insert the tip into the center of each cupcake and gently squeeze to fill.

Melt the bittersweet chocolate in a heatproof bowl set over a pot of simmering water, stirring until smooth. Remove from heat and allow to cool for 20 to 30 minutes. In a stand mixer fitted with the paddle attachment, beat the cream cheese and butter on medium-high speed until pale and fluffy, approximately 3 to 4 minutes. Gradually add the sifted confectioners’ sugar, cocoa powder, and salt, mixing until fully incorporated. Add the melted chocolate and sour cream, continuing to beat until the frosting is smooth and well-blended.

Transfer the buttercream to a piping bag fitted with a large star tip. Pipe generous swirls of frosting onto the filled cupcakes, beginning at the outer edge and spiraling inward.