Preheat the oven to 350°F and prepare a cupcake pan with liners.
Place the dark cocoa powder into a small, heatproof bowl, and pour the boiling water over it. Whisk these together until smooth and set it aside to cool until just warm to the touch.
In a separate, larger bowl, whisk together the sugar, oil, egg, and vanilla extract until smooth. Then add the yogurt, milk, and cooled chocolate mixture until well combined.
Sift the flour, baking soda, baking powder, and salt together, and then add them to the wet ingredients. Use a flexible spatula to fold the flour in until it is just combined. The final batter will be quite runny.
Divide the batter evenly between the cupcake tins, filling to just over ¾ full. Bake for 18-22 minutes, or until the cupcakes are set and a toothpick comes out clean. Let the cupcakes cool in the pan for 10 minutes before transferring them to wire racks to cool completely.