Melt the butter with the spring water and sugar in a heavy-based pan. Bring to the boil, turn off the heat and immediately tip in all the flour. Beat vigorously with a wooden spoon until the mixture forms a ball in the centre of the pan. Tip the ball into a bowl and cool for 15 minutes. Whisk in the eggs gradually until the mixture is smooth and glossy.
Sprinkle water over a nonstick baking sheet. Put 8 large spoonfuls of the mixture, well spaced apart, on to the baking sheet. Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 30 minutes until risen and golden brown, then turn off the oven. Pierce each bun with a sharp knife to let the steam escape and leave in the oven for 10–15 minutes to dry out. Transfer to a wire rack and cool completely.
Melt the chocolate with the butter in a bowl over a pan of simmering water (see
Whip the cream with the vanilla extract and sugar. Spread a spoonful of dulce de leche (toffee caramel) into the base of each bun and top with a spoonful of whipped cream and the choux bun lids. Spoon the melted chocolate on top.