Put the water and butter in a saucepan and bring to the boil. Sift together the flour, baking powder, sugar and bicarbonate of soda into a bowl, then quickly tip into the pan and beat with a hand-held electric whisk. Gradually beat in the eggs until glossy, then spoon the mixture into a piping bag with a plain nozzle and pipe small mounds on to 2 lightly greased and dampened baking sheets.
Place in a preheated oven, 200°C (400°F), Gas Mark 6, for 10 minutes, then increase the temperature to 220°C (425°F), Gas Mark 7, and cook for a further 12–15 minutes until crisp and golden. Remove from the oven, pierce a hole in the side of each of the profiteroles and transfer to a wire rack to cool.
Make the chocolate sauce. Place the chocolate and cream in a heatproof bowl over a saucepan of simmering water and leave until melted. Stir together and leave to cool a little.
Whip the cream with the icing sugar in a bowl until it forms soft peaks and use to fill each bun. Pile the buns on to a serving dish, then pour over the chocolate sauce.