Preheat the oven to 160C Fan and lightly butter a 1.25-1.5 L oven-proof dish.
In a large mixing bowl, gently whisk together the flour, baking powder, caster sugar, cocoa powder and a pinch of salt until combined.
In a separate bowl, whisk together the eggs, melted butter and milk. Pour into the dry ingredients and mix together to form a smooth batter.
Pour into the prepared baking dish and smooth over the top. Set aside.
Meanwhile, for the sauce: mix together the boiling water, sugar and cocoa powder in a large jug until the sugar has dissolved. Carefully pour over the pudding batter - it will look a bit odd at this point but don’t worry, it is normal!
Bake for 25-30 minutes or until the surface looks firm, risen and crisp - the exact time will depend on the size and shape of your dish. Remove from the oven and leave to stand for 1-2 minutes to settle.
Spoon into bowls, revealing the rich chocolate sauce hidden underneath the sponge (make sure you spoon plenty of this onto each portion). Serve warm with vanilla ice cream and/or cream, if desired.