For the lemongrass parfait, place 150ml/5fl oz of the double cream into a small saucepan with the lemongrass and bring to a simmer. Set aside to infuse for at least one hour. Once infused, blend the mixture in a food processor to a fine purée. Pass the mixture through a fine sieve and chill in the fridge until cold.
Meanwhile, bring a little water to the boil in a pan, then reduce the heat until the water is simmering. Suspend a heatproof bowl over the water (do not allow the base of the bowl to touch the water). Whisk the egg yolks and 75g/3oz of the sugar together in the bowl until pale and fluffy.
Continue cooking, whisking continuously, for 5-6 minutes, or until the mixture thickens.
Remove the egg and sugar mixture from the heat and whisk in the cold lemongrass mixture. Whisk until the mixture has cooked and is pale and fluffy. Chill in the fridge for 1-2 hours until cold.
Whisk the egg whites in a bowl, until soft peaks form when the whisk is removed. Slowly whisk in the remaining sugar until the egg whites are glossy.
Fold the meringue mixture into the lemongrass mixture.
Whisk the remaining 300ml/10½oz of double cream in a bowl until stiff peaks form when the whisk is removed. Fold into the lemongrass mixture.
Place six small chefs' rings onto a baking tray lined with greasproof paper and pour the lemongrass mixture into the rings. Tap the side of the rings gently to remove any air, then chill in the freezer for two hours, or until just solid.
For the honeycomb, place the sugar, honey, liquid glucose and water into a saucepan and bring to the boil. Continue to cook until the temperature reaches 160C (use a sugar thermometer to check this), remove from the heat and whisk in the bicarbonate of soda.
Pour the mixture onto a baking tray lined with greasproof paper. Set aside until the mixture becomes hard, then break into very small pieces using a toffee hammer.
For the toffee apple bananas, heat a frying pan until hot. Add the bananas, cut-side down, and fry for 1-2 minutes, or until caramelised. Remove from the pan and set aside. Add the butter to the pan and heat until the butter foams, then add the brown sugar and cook until the mixture is a caramel colour.
Add the lemongrass and the cream, bring to the boil, then reduce the heat until the mixture is simmering and simmer for 2-3 minutes, or until golden-brown. Return the bananas to the pan and heat through. discard the lemongrass.
To serve, dip the top and bottom of each parfait into the honeycomb pieces.
Using a warm cloth or a chefs' blowtorch, heat the sides of each chefs' ring to loosen the parfait and turn them out onto the centre of six serving plates. Place a couple of pieces of banana on the side of each parfait then spoon the lemongrass toffee sauce over the top.
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