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1
Bring pot of salted water to boil with vermouth; add prawns and cook 3 minutes until pink and firm (prawn flesh should just opaque at thickest point).
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2
Transfer prawns to ice bath immediately; drain thoroughly and chill 10 minutes.
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3
Season chilled prawns with lemon juice, salt, and white pepper.
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4
Arrange Cos leaves around interior of four chilled coupe glasses or shallow bowls to create cup shape.
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5
Pile equal portions of chilled prawns into center of each glass; coat generously with Marie Rose sauce.
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6
Serve immediately with toasted bread and lemon wedges on the side.