Preheat oven to 350 degrees F. Line a cupcake tin with paper liners.
In a medium bowl, whisk cocoa powder and baking soda together. Pour in boiling water and stir until well blended. Cool.
In a large mixing bowl, combine melted butter, vegetable oil, white sugar, vanilla, and salt. Stir in egg and egg yolk.
Pour in heavy cream and stir in cocoa powder.
Gradually stir in flour a little at a time until just combined. Do not overmix!
Spoon ~2 Tablespoons of batter into each of the 24 cupcake liners.