Place cream and lemon rind in a small saucepan and stir over low heat until just warm, then add lemon juice and butter and stir until butter is melted. Season to taste with sea salt and freshly ground black pepper, then remove from heat. Stir in egg yolk and combine well. Cook pasta in boiling salted water until al dente, then drain well and return to same pan. Add warm cream mixture, prawns, prosciutto and half the chervil, then toss to combine well. Divide pasta among warm plates, scatter with remaining chervil and serve immediately.