1.Preheat oven to 180°C. Grease a 20cm x 30cm rectangular pan; line base and long sides with baking paper, extending paper 5cm over sides.
2.Stir chocolate and butter in a small saucepan over low heat until smooth. Transfer mixture to a large bowl and cool for 10 minutes.
3.Stir in sugar, egg, sifted flour and weet-bix. Spread mixture into pan; smooth surface. Bake about 10 minutes. Cool slice in pan.
4.To make chocolate glaze, sift icing sugar and cocoa into small bowl. Add enough of the hot water to make glaze spreadable.
5.Spread slice with glaze; stand at room temperature until set before cutting into pieces.