Heat vegetable oil in a deep frying pan until a cube of bread sizzles and turns golden.
Whisk flour, cornflour, sugar, and sparkling water in a bowl to create a thick batter.
Coat bananas in batter and fry in hot oil until golden-brown and crisp.
Drain fried bananas on kitchen paper.
Melt dark and white chocolates in separate bowls over simmering water.
Whip double cream until soft peaks form.
Fold two-thirds of the cream into the dark chocolate and the rest into the white chocolate.
Transfer mixtures to piping bags.
Pipe layered chocolate mixtures into 5cm rings on greaseproof paper and chill for 1-2 hours.
Press the base of chilled mousses into chopped hazelnuts and place on plates.
Remove rings, add two pieces of banana to each plate, and top with Space Dust.