Preheat the oven to 180°C (356°F).
Sift and combine cake flour, cocoa powder, and baking powder.
Cream the butter and sugar together until smooth.
Add the eggs and mix thoroughly.
Melt the chocolate mousse ingredients (dark chocolate, heavy cream, etc.) in a double boiler and stir until smooth.
Add the dry ingredients to the butter mixture and mix them together.
Fold in the chocolate mousse and chocolate chips into the batter.
Spread the batter in a pan and top with the crumble topping (a mixture of butter, sugar, and flour).
Bake in the oven at 180°C (356°F) for 20–25 minutes.
Let cool, then cut into pieces for serving.