Preheat oven to 350°F (177°C). Line a 12-cup muffin tin with cupcake liners and set aside.

In a large mixing bowl whisk together 1 cup (200 g) sugar, 1 cup (125 g) all-purpose flour, 1/2 cup (60 g) dark or Dutch-processed cocoa powder, 1 teaspoon baking soda, 1/2 teaspoon baking powder, and 1/2 teaspoon kosher salt until evenly combined.

In a separate bowl whisk together 1 large egg (at room temperature), 1/2 cup buttermilk, 3 tablespoons hot brewed coffee (or plain hot water), 1/3 cup vegetable or light olive oil, and 2 teaspoons vanilla extract until blended.

Add the wet ingredients to the dry ingredients and beat with an electric mixer on medium speed just until the batter is smooth and the dry ingredients are incorporated (do not overmix). Scrape down the bowl as needed.

Using a large cookie scoop or a spoon, portion about 3 tablespoons of batter into each lined cup, filling no more than 2/3 full.

Bake for 16–18 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs attached.

Remove cupcakes from the oven and let the pan sit on a wire rack for about 5 minutes, then transfer the cupcakes to the wire rack to cool completely. Do not frost while cupcakes are still warm.

While cupcakes cool, melt 5 ounces dark chocolate (60–70%) and let it cool to lukewarm. (Melt gently in short microwave bursts, stirring between each, or in a heatproof bowl set over simmering water.)

In the bowl of a stand mixer fitted with the paddle attachment, beat 1 cup (2 sticks) unsalted butter (softened) on medium speed until very pale and creamy, about 5 minutes.

Add 1 1/4 cups (150 g) sifted powdered sugar and 1/2 cup (60 g) sifted dark or Dutch-processed cocoa powder to the butter. Mix on low speed until the dry ingredients are incorporated, scraping down the bowl as needed.

Add 1/4 cup full-fat sour cream (at room temperature), 3 tablespoons hot water, 1 teaspoon vanilla extract, and a pinch of fine sea salt. Mix on low speed until the liquid is fully incorporated; the mixture may look slightly curdled.

Increase the mixer speed to medium-high and beat until the frosting is smooth and fluffy, about 2–3 minutes.

Reduce mixer to low and add the lukewarm melted chocolate. Mix until the frosting is smooth and glossy.

Spread or pipe the frosting onto the completely cooled cupcakes.

Store cupcakes at a cool room temperature. Refrigeration will cause the frosting to firm/harden; if you do refrigerate, allow cupcakes to come back to room temperature before serving.