For the tempura bananas, pour in enough vegetable oil to come two-thirds of the way up the sides of a deep, heavy-bottomed frying pan and heat until a small cube of bread sizzles and turns golden when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Place the flour, cornflour and sugar into a bowl and gently whisk in enough sparkling water to make a thick batter - it doesn't matter if it isn't completely smooth.
Dip the bananas into the batter to coat completely, shaking off any excess, then place into the hot oil. Fry the bananas until golden-brown and crisp. Remove from the pan with a slotted spoon and drain on kitchen paper.
For the mousse, place the dark chocolate and white chocolate into separate heatproof bowls set over pans of gently simmering water and heat, stirring occasionally, until smooth and melted.
Meanwhile, lightly whip the cream until soft peaks form when the whisk is removed.
Gently fold two-thirds of the cream into the melted dark chocolate until well combined, then spoon into a piping bag and set aside.
Repeat with the remaining cream and the white chocolate and spoon into another piping bag.
Place four 5cm/2in chefs' rings onto a large plate lined with greaseproof paper. Pipe a layer of the dark chocolate mixture, followed by the white chocolate mixture into each ring. Repeat until the rings are full, then smooth the tops using a palette knife. Place into the fridge to set for 1-2 hours.
To serve, remove the mousses from the fridge and remove the greaseproof paper. Press the base of each ring into the chopped hazelnuts to coat, then place onto serving plates. Carefully lift off the chefs' rings and place two pieces of banana onto each plate. Sprinkle the Space Dust over the top of each mousse.