Using a food processor, blend the oats into a fine flour. Grease and line the baking mould with parchment paper. Preheat the oven.
Top/Bottom Heat: 180°C
Fan Assisted: 160°C
Mix the oats and the other ingredients until combined. Firmly press dough into the bottom of the pan. Bake and let it cool completely.
Rack position: Middle rack
Baking time: about 15 Minutes
Melt the dark chocolate in a small heatproof bowl over a saucepan of barely simmering water. Blend the remaining ingredients. Stir in the melted chocolate and mix thoroughly. Set aside 4 tablespoons of the mousse mixture and spoon into a piping bag fitted with a star nozzle. Pour the rest of the mousse filling into the mould.
With the remaining mousse mixture, pipe small swirls round the top edge of the cake. Let the cake chill for about 2 hours in the fridge, before serving. Enjoy!