Preheat oven to 350 °F/175°C. Line standard muffin tins with paper liners. Mix together cake flour, cocoa, and salt.

With an electric mixer on medium-high speed, whisk together sugar and oil until combined. Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed. Mix in vanilla extract.

Reduce speed to low. Add flour mixture in three batches, alternating with two additions of buttermilk, and whisking well after each. Stir together the baking soda and vinegar in a small bowl (it will foam); add mixture to the batter, and mix on medium speed 10 seconds.

Pour batter into lined cups, filling each 3/4 full. Bake until a toothpick inserted in centers comes out clean, about 20 minutes. Transfer tins to wire racks to cool completely before removing cupcakes.

In the bowl of an electric mixer with the whip attachment, beat together the butter and salt. Add the confectioners’ sugar and cocoa powder. Mix on low speed until just combined (the mixture may be crumbly at first. Add heavy cream a little at a time until the mixture is piping consistency. Add the dark chocolate to the frosting and whip on high speed. Scrape down the bowl and whip again until light and fluffy.

Transfer the frosting to a piping bag fitted with a large French tip. Pipe frosting onto cooled cupcakes.