Heat the oven to 220C/200C fan/gas 7. To make the profiteroles, put the butter and 2 tsp of the caster sugar in a saucepan with 150ml water. Place the pan over a low heat until the butter and sugar have melted, then bring to the boil. Take off the heat, add the flour all at once and beat energetically with a wooden spoon until the dough comes away from the sides of the pan.
Leave to cool for 5 mins, then beat in the eggs bit by bit until you have a stiff, glossy mixture (this process is much easier in a food processor). Rinse two baking trays with cold water, shaking off any excess so they are slightly damp (this helps the pastry to rise). Using 2 teaspoons, spoon blobs of the mixture onto the baking trays. Then place in the oven and cook for about 18-20 mins until well risen and brown. Remove the profiteroles from the oven and cut a small slit in the base of each one so they don’t collapse. Cool on a wire rack.
When they’re cold, whip the cream lightly until just holding its shape. Sweeten to taste with remaining sugar and a few drops of vanilla extract. Cut the profiteroles in half, fill them with the sweetened cream and pile them up on a plate. You can refrigerate them for 1-2 hrs at this point but not for any longer as the pastry will go soggy.
To make the sauce, sift the cocoa powder into a bowl. Put the sugar in a pan with 100ml water and warm over a low heat until dissolved. Bring to the boil, cook for 1 min, then pour over the cocoa powder and stir well until smooth. Return the sauce to the pan, cook for 1 min then set aside for 15 mins before drizzling over the profiteroles.