Discard the tough outer leaves of the artichokes, cut in half lengthwise and discard the choke. Quarter and place in water with some lemon juice to prevent oxidation.
Melt butter in a wide pan over medium heat, stir in rice starch and cook for a couple of minutes. Add drained artichokes and vegetable broth. Cook for 25 minutes, remove from the heat and purée in a blender. Season with salt and pepper.
Strain the purée through a sieve to remove any strings. Before serving, add egg yolks and Parmigiano to the soup. Stir with a wooden spoon to combine and serve warm. Garnish with some crostini sprinkled with extra virgin olive oil.