Preheat the oven to 350°F. Line 24 cupcakes tins with baking papers. Place the chocolate, butter, and boiling water in a medium, heatproof bowl placed over a pot of gently simmering water. When the chocolate and butter are just melted, whisk in the sugar. Let cool to lukewarm temperature, about 4 minutes.

Whisk the eggs into the mixture. Add the buttermilk, vanilla, salt, and baking soda; whisk to combine. Whisk in the flour until evenly combined.

Fill each baking paper two-thirds full. Bake until a skewer inserted into the middle of a cupcake comes out clean, about 20 minutes.