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Mix the flour, mustard powder, cayenne pepper, rosemary and a generous grind of black pepper together until well-combined. Beat in the butter with a wooden spoon until the mixture resembles a smooth paste, then stir in the cheeses and egg yolk until evenly combined.
Shape the dough into a 30cm log, rolling it back and forth on a clean work surface using the palms of your hands until it is an even thickness. Sprinkle the poppy seeds over a large sheet of baking parchment, put the log on top and roll back and forth until evenly coated. Wrap the log in the parchment, twist the ends and chill in the freezer for 35 mins until firm but not frozen.
Heat the oven to 190C/170C fan/gas 5 and line two large baking trays with baking parchment. Unwrap the log, trim the ends and slice it into 1cm rounds. To get ahead, freeze the slices on lined baking trays, then tip into a freezerproof bag and freeze for three months (see tip, below). Place the rounds 3cm apart on the trays and bake for 12-15 mins until golden and slightly mottled on top. Leave to cool completely on the trays. Will keep in an airtight container for up to two weeks. Crisp in a hot oven if needed.
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