Combine the mascarpone cheese, egg, sugar, salt, vanilla, and chocolate chips in a small bowl and whisk lightly to combine. Place in the freezer, for at least an hour, stirring every fifteen minutes or so, until scoopable.

Preheat the oven to 350 degrees and line a cupcake pan with paper liners. Combine the sifted flour, cocoa powder, and salt in a medium sized bowl and whisk. Set aside. Place the butter and chocolate in a small microwave-safe bowl, and microwave on high, for 20 second intervals, stirring after each interval, until melted. Alternatively, place the butter and chocolate in a small sauce pan and melt over medium heat on the stove-top, stirring often.

Place the warm butter/chocolate and sugar in the bowl of a stand mixer fitted with the paddle attachment and on medium speed, beat them together until combined, scraping the bowl with a rubber spatula, as needed. add the egg, then yolk, then vanilla, mixing after each addition and continuing to scrape as needed.

On medium low speed, add the flour/cocoa powder/salt to the mixing bowl, in three additions, alternating with the buttermilk, and beginning and ending with the dry ingredients. The first addition of dry should be the largest (add more than 1/3 of the dry ingredients initially). Stop the mixer and remove the bowl when there are still streaks of flour throughout the batter. Use a rubber spatula to incorporate the streaks.

In a small bowl, combine the vinegar and baking soda, whisking with a fork to dissolve the baking soda. Work quickly, and once the baking soda is dissolved and the mixture is furiously bubbling, add it to the batter, bubbles and all. Stir with a rubber spatula to incorporate.

Place 1 1/2 tbsp of batter in each paper lined cup (I used a 1 1/2 tbsp ice cream scoop). Add a 1 1/2 tbsp scoop of mascarpone filling (using a small offset spatula to press the filling down into the batter, if necessary) and add a final 1 1/2 tbsp scoop of batter. Use the offset again to smooth the top layer of batter over the mascarpone (easier said than done, but do your best).

Place the cupcakes in the oven and bake for 22 to 24 minutes, rotating after 12. Begin checking on the cupcakes at 20 minutes, sticking a cake tester into the cupcakes (avoiding the filling if possible) until the tester comes out with a few moist crumbs. The cupcakes will sink (see my pictures) so do not be alarmed.

Place the cupcakes, still in the pan, on a wire rack and cool until you can remove them with your hands. Serve while still warm on plates with forks (they can be a bit messy at this stage — but, oh so good). The cups will keep at room temperature, covered in plastic wrap for a day or two, but are best eaten the day they are made.