Preheat the oven to 425°F / 220°C and line baking sheets with parchment paper. Measure 2 tablespoons of superfine sugar.
Bring 50g of butter, a third of the sugar, and 150ml of water to a boil. Off the heat, add 75g of bread flour and 1 pinch of salt all at once and beat until smooth.
Cool for 5 minutes, then gradually beat in 2 beaten eggs until glossy. Using two teaspoons, drop small mounds onto the baking sheets, spaced apart.
Bake for 18-20 minutes, until well risen and brown. Cut a small slit in the base of each bun and cool on a wire rack.
Dissolve 175g of superfine sugar in 100ml of water over low heat; boil for 1 minute. Stir into 50g of sifted cocoa powder until smooth, then simmer briefly. Let stand for 15 minutes.
Lightly whip 300ml of heavy cream to soft peaks, then fold in the remaining sugar and 3 drops of vanilla extract. Halve the buns, fill with the cream, and drizzle with the sauce.
