Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners and set aside.
In a large mixing bowl, whisk together the flour, cocoa powder, sugar, baking soda, baking powder, and salt until no lumps remain and the mixture is evenly combined.
Add the egg, sour cream, vegetable oil, and vanilla extract to the dry ingredients. Whisk vigorously for about 30 seconds until a thick batter forms.
Pour in the hot coffee (or hot water) and whisk gently until the batter is smooth and pourable, about 15 seconds. The batter will be quite thin — this is correct.
Divide the batter evenly among the 12 liners, filling each about 2/3 full. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. Do not overbake.
Remove the cupcakes from the oven and let them cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting, about 1 hour.
While the cupcakes cool, make the buttercream: Beat the softened butter in a large bowl with an electric mixer on medium speed until creamy and smooth, about 2 minutes.
Add the sifted powdered sugar and cocoa powder to the butter in two additions, beating on low speed after each addition until mostly combined. Scrape down the sides of the bowl as needed.
Pour in the heavy cream, vanilla extract, and salt. Beat on low speed for 30 seconds to incorporate, then increase to medium-high and beat for 3-4 minutes until the buttercream is light, fluffy, and silky smooth.
Frost the cooled cupcakes using a piping bag fitted with your favorite tip, an offset spatula, or the back of a spoon. Top with sprinkles, chocolate shavings, or a dusting of cocoa powder if desired.
