Prepare the mousses in advance. Melt the chocolates in the cream for each of the mousses, using a non-stick pan, then cover and let rest overnight.
The next day, prepare the base by melting the dark chocolate with the butter in a bowl over simmering water, leaving it to cool, then beating the eggs with the sugar. Add the chocolate to the egg mixture, then gradually mix in the flour. Transfer into a greased tin, sprinkle with chopped milk chocolate.
Bake in the oven at 180 C for 30 minutes. Leave to cook on a wire rack.
When cooled down completely, cut off the edges Top with the mousses, interlacing them gently or keeping one on top of the other, then sprinkle with the brownie "debris".