Chocolate Frosted Traybake

As promised here is the recipe I found on the Pink Whisk website, it's scrummy and very simple to make so give it a try!
 
Ingredients:

Cake
110g butter
120ml boiling water
2 heaped tablespoons of cocoa powder
190g caster sugar
130g plain flour
1/2 tsp bicarbonate of soda
60ml milk
1 egg, medium

Frosting
100g butter
2 heaped tablespoons cocoa powder
2 tbsps milk
200g icing sugar, sifted

Method
Line an 8"square tin with baking paper (no need to be neat).
Preheat the oven to 160c (fan)/180c/Gas Mark 4.
Place the butter, boiling water and cocoa powder in a saucepan and heat it over a medium heat until the butter melts and it begins to bubble. Bubble the mixture for 30 seconds and then take it off the heat.
In a bowl mix together the flour, caster sugar and bicarbonate of soda, mixing them now should stop you getting any lumps of flour. Add this dry mixture to the pan and give it a good stir .
Add the milk and egg to the pan and mix it well again then pour into the lined tin.
Bake in the oven for 20 mins.
Remove from the oven and set aside to cool in the tin.
Once cooled it’s time for the frosting.
Put the butter and cocoa powder in a saucepan and heat gently to melt the butter, and then allow the mixture to have a good bubble for 30 seconds to zap off any extra moisture, give it a good stir and remove from the heat.
Stir in the milk and then add the sifted icing sugar and mix until smooth, then pour over the cake still in the pan.
Leave the frosting to set and once set, cut into slices and enjoy!!