Mix the buckwheat, oats, sugar and butter.
Approx. 30 mins. in the centre of an oven preheated to 180°C, stirring occasionally with a fork. Remove the crumble from the oven, leave to cool.
Salted butter caramel
Boil the sugar and water in a pan without stirring. Reduce the heat and simmer, swirling the pan occasionally until a light brown caramel has formed. Add the butter and stir to melt.
Pour in the cream and simmer, stirring occasionally for approx. 5 mins. until the caramel thickens. Pour the caramel into the glasses, chill for approx. 1 hr.
Heat half of the cream in a pan, remove the pan from the heat, add the chocolate, cover and melt, stir until smooth, leave to cool. Beat the remainder of the cream until stiff, carefully fold into the chocolate using a rubber spatula. Spread the mousse over the caramel, cover and chill for approx. 2 hrs.