Preheat oven to 180C/350F/Gas 4. Line the bottoms of two 20cm/8cm sandwich tins with baking parchment and butter the sides.
Place the cocoa and dark brown sugar in a large bowl and pour in the boiling water. Whisk to blend, then set aside.
Cream the butter and caster sugar together in a separate bowl, beating well until pale and fluffy. I find this easier with a stand-alone mixer, but by hand it wouldn’t kill you.
Mix the flour, baking powder and baking soda together in another bowl and set aside for a moment.
Dribble the vanilla extract into the creamy butter and sugar – mixing all the time – then break into one egg, followed quickly by a spoonful of the flour mixture, then the second egg. Continue mixing after each addition.
Incorporate the rest of the flour mixture little by little, then mix and fold into the cocoa mixture, scraping the bowl well with a spatula.
Divide this fabulously chocolatey batter between the two prepared tins and place in the oven for about 30 minutes, or until a cake tester comes out clean. Take out the tins and leave them on a wire rack for 5-10 minutes, then turn the cakes out and set aside to cool.
Once the cakes are in the oven, start freezing. Place the water, brown sugar and butter in a saucepan over low heat until melted.
When this mixture starts to bubble, remove the pan from the heat and add the chopped chocolate, swirling the pan so that all the chocolate is hit with heat, then leave for a minute to melt. Once melted, beat until smooth and shiny.
Set the frosting aside for about an hour, beating occasionally – when passing through the pan – when the cakes will be cooled and ready for frosting.
Place one of the cooled cakes, top side down, on a cake stand or plate, and spread with about a third of the frosting. Top this with the second cake, evenly on top, and spread the remaining frosting over the top and sides, turning with your spatula. You can take a soft look, but I never do and probably couldn’t.