Lightly oil a 20cm square shallow tin
Melt the chocolate very slowly in a microwave making sure not to burn it. If you prefer the traditional way. Melt the chocolate in a heatproof bowl set over a saucepan of barely simmering water, making sure that the surface of the water does not touch the bowl.
Place the water, glucose and sugar in a heavy based saucepan and heat over a medium heat until it forms a nice blond caramel. Remove from the heat and add 15g of the butter to the cooled mixture.
Put the cream into a small sauce pan and heat slightly, then slowly pour it into the caramel, stirring gently. Add the remaining butter, return the pan to the stove and heat until the mixture reaches 118 deg C on a sugar thermometer.
Remove from the heat and stir in the melted chocolate until smooth. Pour onto the prepared tin and leave to cool overnight at room temperature.
Turn the toffee out onto a chopping board and cut into small squares using a large knife.
TIP: Lay each toffee on top of the tangs of a fork and dip them into 52% dark couverture chocolate. As long as it is tempered they will set beautifully.