Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" cake tin.
In a large bowl, or the bowl of a stand mixer, combine the flour, baking powder, baking soda, cocoa powder, sugar and salt.
Get the kettle on – essentially we are going to make a hot chocolate. Combine the boiling water, hot chocolate powder and milk in a mug. Set aside.
Make a well in the dry mix, and add the eggs, syrup and oil. Once thoroughly combined, slowly pour in the hot chocolate mixture until it has all been incorporated. The batter will be fairly thin.
Pour the batter into the tin and bake for 35-40 minutes, until a skewer inserted comes out clean. Let cool on a wire rack.
Once cool, make the frosting: beat the butter on high with an electric mixer. Add half the icing sugar and the melted chocolate and combine for several minutes until combined. Add the remaining icing sugar until you reach your desired consistency.
Spread over the cake, add some chocolate decoration if you like (I grate dark chocolate and add any kind of chocolate I have in the cupboards)! Slice and serve.