1.Stir butter, sugar and the water in a medium heavy-based saucepan over low heat until sugar is dissolved and butter melted. Bring to the boil, stirring occasionally, until toffee mixture reaches 155°C on a sugar thermometer.
2.Remove pan from the heat; allow bubbles to subside. Stir in extract. Drop tablespoonfuls of toffee mixture onto baking-paper-lined trays. Stand until set.
3.Melt chocolate. Dip toffee into chocolate, then in nuts; return to trays. Refrigerate until chocolate is set.
If you prefer, you can make one large slab of toffee; pour the toffee mixture onto a baking paper-lined oven tray and stand until set. Spread the back of the toffee slab with melted chocolate then sprinkle with the nuts. Refrigerate until the chocolate is set, then break toffee into pieces before serving. You will need a sugar (candy) thermometer for this recipe.