Preheat the oven to 200C/180C Fan/Gas 6. Line a 2 lb loaf 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.
Meanwhile, start to make the frosting (as it needs time to cool before whisking). Finely chop the chocolate and put in a heatproof bowl. Heat the cream on the hob on a low heat, until it just starts to bubble. Pour over the chopped chocolate and leave for 2 minutes.
After those two minutes, stir together until a gorgeously shiny chocolate ganache forms - it should all be one chocolatey colour. Let cool (at least 1 hour).
At this stage, everything should be completely cool. Add the chocolate ganache to a stand mixer, or a bowl that you can use an electric mixer in. Whisk on a high speed for 2-3 minutes, until light in colour and fluffy.
Spread this over the top of the cake, adding any sprinkles/decorations you'd like! Serve as soon as you can!