Heat the chocolate in a bowl over a pan of simmering water.
Meanwhile, put the hobnobs in a bag, and bash them repeatedly with a rolling pin until you’ve got a bagful of crumbs. Stir into the melted chocolate.
Set 2 rosti rings on a non-stick tray, and press the chocolate-biscuit base into the bottom of each, then put in the fridge so the chocolate sets.
In the meantime, thinly slice the strawberries. When you take the bases out of the fridge, arrange the strawberry slices in a circle around the inner edge of the rosti ring.
To make the mousse, melt the white chocolate in bowl over a pan of simmering water. Then stir in the crème fraiche and double cream. Pour this into the centre of the 2 moulds. Put everything back in the fridge, and leave it to set for 4 hours.
Serve with extra strawberries, or a drizzle of strawberry coulis and sprig of mint.
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