Preheat oven to 180°C
Line a 33cm x 23cm slice pan or brownie pan with baking paper. Set aside.
Place butter in a small saucepan over low heat until melted.
Sift flour and cocoa into a large bowl.
Add sugar and coconut to the flour and cocoa and combine.
Add melted butter to the flour mixture and stir until well combined.
Press mixture firmly and evenly over the base of the prepared pan.
Bake for 25-30 minutes or until the mixture is starting to bubble around the edges. It may feel soft at this stage but it will become firm as it cools.
Cool in pan and then place on a wire rack. Refrigerate the slice prior to icing, this will make it easier to ice.
Place butter and hot water in a small saucepan over low heat until the butter is melted.
Sift icing sugar and cocoa powder into a bowl.
Add melted butter to icing sugar mixture and stir until smooth. Add small amounts of extra hot water and continue to mix if the icing seems too dry.
Spread icing evenly over cold slice and top with coloured sprinkles. Cut into pieces to serve.
Store in a airtight container. Refrigerate in warm weather.