Stir in the sugar and the coffee essence and knead to a smooth dough.
Press the dough in a even layer into a lined tin (8″ square), prick with a fork and bake at 350F/180C for 20-25 minutes. Leave to cool.
Place all the caramel ingredients in a heavy based pan. Heat gently until the butter and sugar have melted. Bring to a boil and simmer for 5-7 minutes, stirring frequently, until it thickens.
Allow to cool for a few minutes, then pour over the base and leave it to firm up for a while.
Melt the chocolate and butter, either in a bowl over a pan of simmering water or in short bursts in the microwave. Spread over the base and leave to set.