For the ice cream, place the egg yolks and sugar in a bowl and whisk together until combined.
Place the milk and lavender into a pan and bring to the boil.
Pour the boiling milk onto the egg yolks and whisk well, then pour back into the pan and return to the heat.
Add the cream and continue to cook until the mixture has thickened, but do not allow to boil.
Place the white chocolate into a large heatproof bowl. Pour three quarters of the lavender custard mixture through a sieve onto the white chocolate and stir well. Allow the mixture to cool, then place into an ice cream machine to churn to ice cream.
Place the remaining quarter of the custard into a clean bowl and transfer to the fridge to chill.
For the fondant, preheat the oven to 180C/355F/Gas 4.
Grate 40g/1½oz of the dark chocolate into a bowl and set aside.
Melt half of the butter and brush liberally around the inside of four ramekins. Dust the ramekins well with the grated chocolate, shaking out any excess, then place onto a baking sheet and set aside.
Place the remaining chocolate, including any shaken out excess, and the remaining butter into a heatproof bowl and place over a pan of barely simmering water to melt.
Add the ground almonds, egg yolks and cornflour to the melted chocolate and stir well.
In a separate bowl, whisk the egg whites until stiff peaks form when the whisk is removed.
Gradually add the caster sugar, whisking constantly.
Gently fold the egg white mixture into the melted chocolate mixture, taking care to retain volume in the whites.
Spoon half the chocolate and egg mixture into the base of each prepared ramekin. Place a chocolate truffle on top, then fill each ramekin with the remaining mixture.
Smooth the tops of the fondants with a palette knife, then transfer to the oven to bake for 10-15 minutes, until risen (the surface should be slightly wobbly), then remove from the oven and serve immediately.
To serve, strain the reserved, chilled custard mixture and place a spoonful onto the centre of each of four plates. Turn out the chocolate fondants and place into the centre of the sauce. Dust the top of the fondant with the cocoa powder and place a spoonful of lavender ice cream alongside.
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James Martin presents a compilation of favourite Saturday Kitchen moments.