Special equipment: 6 fairly heavy China coffee cups
Make the Cake: Preheat the oven to 350 degrees F.
Sift together the sugar, flour, cocoa, baking powder, baking soda, and salt. Transfer to a standing mixer fitted with a whisk attachment (or use a hand mixer) and blend briefly.
Whisk together the eggs, milk, oil, and vanilla in a medium bowl. Add to the dry ingredients and mix at low speed for 5 minutes. Gradually add the hot water, mixing at low speed until just combined. The batter will be quite thin.
Pour the batter into coffee cups and arrange them 1-inch apart on a sheet pan or a rectangular cake pan. Bake until a toothpick inserted in the center comes out clean (a few crumbs are okay), and the center feels firm to the touch, about 25 to 30 minutes. Let the cakes cool on the pan. (Don't remove the cakes from the cups!)
Meanwhile, make the Topping: In a chilled bowl, combine the cream and sugar and whip, using a hand-mixer, until soft peaks form.
When ready to serve, spoon the topping over the cakes to cover, so they look like a cups of cocoa topped with whipped cream. Serve with a spoon.