Pre-heat the oven to 350℉/ 180℃/ 160 fan
Grease and line a 7 inch/ 18 cm spring form tin (See recipe notes)
In a stainless steel bowl add your cocoa powder and boiling water and mix to a paste with a hand held mixer or a hand whisk
Add the soft butter cut into small pieces and mix well
Add the eggs and mix well
Add the sugar, flour and baking powder and mix with the whisk and then a spatula
Place your prepared spring form tin on a baking tray and, using the spatula, pour the cake mixture into the tin
Encourage the cake mix into the edges with the spatula and level and smooth the top but no need to be too fussy
Bake for 23-25 minutes, keeping the door closed until you need to check it. Dip the cake with a skewer or something similar and if the skewer comes out clean, the cake is done. If the melty cake coats the skewer, cook for a further two minutes and test again.
Remove from the oven and place to the side to cool for 5-10 minutes. Remove the tin and paper and place the cake upside down on a wire rack to cool properly. Cooling it upside down will help the final texture of the cake
When the cake has cooled enough to handle, carefully cut a thin slice off the top to create a flat base for the mousse. If using the brandy, brush it onto this cut surface at this time and allow to cool completely