Sophie Grigson's Tiramisù with San Marzano

Ingredients

• 500g mascarpone
• 100ml espresso or very, very strong cafetière coffee
• 2 tablespoons Licore di San Marzano or Marsala or Strega or rum
• 2 tablespoons grappa, or Corretto Coffee-Infused Grappa, or brandy
• 4 egg yolks
• 3 egg whites
• 2 pinches of salt
• 70g caster sugar
• 20-24 savoiardi biscuits or ladies’ fingers/sponge finger biscuits
• Unsweetened cocoa powder
• You will also need a 28cmx 20cm baking tin or dish