Preheat the oven to 200C/180C Fan/Gas 6. Line a 3 x 8" circular cake tins.
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.
Split the batter between the three tins and bake for 30-35 minutes, until a skewer inserted comes out clean. Let cool on a wire rack - the cakes should be completely cool before the next steps.
Meanwhile, start to make the ganache: Put the Nutella in a large (heatproof) bowl. Heat the cream in a microwave-safe bowl, in 20 second intervals, until steaming but not boiling.
Pour the hot cream over the Nutella and let sit for 2-3 minutes. Add the icing sugar and whisk the mixture with an electric/stand mixer on high until combined. Put in the fridge for 20 minutes to let set.
Make the buttercream: Add the softened butter to a large bowl/bowl of a stand mixer. Mix on high for a few minutes, to whip the butter. Add the Nutella and mix again until thoroughly combined.
Add in half of the icing sugar and all of the cocoa powder. Once combined, add in the cream and remaining icing sugar. This should come together to form a rich, chocolatey buttercream.
Layer up the cake: start by using one of the cake layers, then scoop a few tbsp of the Nutella ganache into the centre. Create a border at the edge with the buttercream to prevent the ganache from overflowing (it will be a little runny at room temp).
Repeat with the next layer and top with the final cake layer. Spread the buttercream over the rest of the cake before letting chill in the fridge. If desired, add another buttercream layer.
At this stage, decoration is optional. I pipe buttercream swirls on the top and use a spoon to spread some of the ganache around the bottom of the cake, to stick the sprinkles to. You could top with other chocolate/candy/decoration of your choice. Keep in the fridge until ready to serve!