Add butter and sugar in a pan until the sugar caramelises, add 100 ml of cream and simmer over medium heat. Allow to cool.
Dissolve 3 teaspoons of soluble coffee powder with 3 tablespoons of cold water. Cut the brioche into thin slices and place in a bowl, drizzle coffee. Peel bananas, cut into thin slices and mix with grapefruit juice. Spread on the brioches.
Add 2 tsp caramel sauce to the remaining coffee, stir and leave aside. Mix the remaining caramel sauce with mascarpone, stir until cream is stiff. Mix the remaining coffee with 1-2 teaspoons and whip with the mascarpone cream. Pour on to bananas. Decorate with caramel sauce and leave trifle in the refrigerator for at least 30 minutes to set.