Preheat oven to 350ºF. Line a 12-cup capacity muffin tin with cupcake liners and set aside.In a large mixing bowl, combine the sugar, flour, cocoa, baking soda, baking powder and salt, whisking until thoroughly combined. Add eggs, buttermilk, water, oil, and vanilla and beat with an electric mixer on medium speed until all dry ingredients are incorporated. Scoop about 3 tablespoons (a large cookie scoop is the perfect size) into each cupcake tin, filling them no more than 2/3 full.Bake for 16 to 18 minutes or until a toothpick inserted in the center comes out with just a few moist crumbs attached. Remove from oven and place on a wire rack until completely cooled (do not try to frost still-warm cupcakes… trust me!)To prepare frosting, in the bowl of a stand mixer fitted with the paddle attachment, beat butter on medium speed until very pale and creamy, about 5 minutes. Add sifted powdered sugar and cocoa powder and mix to combine. Add sour cream, hot water, vanilla and salt and mix on low speed until the liquid is incorporated, scraping down the sides of the bowl as needed. It may look slightly curdled at this point, but don’t worry. Increase speed to medium-high and beat until smooth, another 2-3 minutes. Add lukewarm chocolate and mix until smooth and glossy.Spread or pipe onto cooled cupcakes. Store at cool room temperature (refrigerating will cause frosting to harden).