To make the cake, soften the butter or
margarine and work in the sugar, flour, egg yolk and water. Spread the
mixture evenly in a 20 cm/8 in square microwave dish and microwave on
High for 2 minutes. Sprinkle over the chocolate and microwave on High
for 1 minute. Spread evenly over the base and leave until hardened.
To make the topping, microwave the
butter or margarine on High for 30 seconds. Stir in the remaining
topping ingredients and spread over the chocolate. Microwave on High
for 5 minutes. Leave to cool, then cut into squares.