Preheat oven to 160℃ and line the base and sides of a 20cm round cake tin with baking paper.
Roughly chop the butter into pieces and place into your Thermomix bowl. Cook for 3 minutes, 60 degrees, speed 2 or until melted.
Add the remaining ingredients and mix for 15 seconds, speed 5.
Scrape down the sides of your Thermomix bowl and mix for a further 15 seconds, speed 5 to completely combine.
Transfer the mixture to your prepared cake tin and bake for 60 minutes or until cooked through when tested with a skewer.
Allow the cake to cook in the tin for 15 minutes before carefully turning out onto a wire cooling rack to cool completely.
Once the cake has cooled completely, you can make the chocolate icing. Sift the cocoa and icing sugar together into a bowl.
Add the softened butter and pour the boiling water directly over the butter.
Using a small whisk, combine all the ingredients until the chocolate icing is smooth. If you find your icing is still a little firm, add an extra teaspoon of boiling water.
Use a large spoon to scoop the icing and place onto your cake. Spread the icing out using a spatula or butter knife until the cake is covered. Decorate with coloured sprinkles.