For the posset: Mix together the cream and 60g sugar in a saucepan and set over a low heat. Gradually heat the mixture to boiling point, then continue to simmer for 3 minutes.
Take the pan off the heat and strain the mixture into a bowl. Leave to cool for a while (I left it for 30 minutes).
Zest one of the blood oranges using a fine grater. Squeeze the juice into the cooled cream mixture and add the zest. Whisk together thoroughly. Pour into espresso cups or shot glasses and chill in the fridge for 2-3 hours.
For the biscuits: Cream together the butter and 75g sugar. Add the rosemary, the zest of the remaining blood orange and the vanilla and work into the creamed mixture. Stir in the egg yolk until you have a nice smooth mixture.
Sieve together the flour, cornflour, baking powder and salt. Gradually work these dry ingredients into the creamed butter and sugar. Add the juice of the remaining blood orange and bring everything together to form a dough. Wrap in cling film and rest in the fridge for 20-30 minutes.
Preheat oven to 180 degrees C. Line a baking sheet with parchment or a non-stick baking mat. Flour your work surface and roll out the dough to 1/2 cm thick. Cut out rounds using a 2" cookie cutter - you should get about 25-30. Brush the tops lightly with egg white and bake for 12-15 minutes until golden and crisp.