Make Curd: Whisk egg yolks, sugar, lemon rind and juice, and salt in a saucepan. Cook, stirring constantly, over low heat for 10 to 12 minutes or until mixture has thickened and coats the back of a wooden spoon. Remove from heat. Add butter, stirring until melted. Strain immediately through a sieve into a glass or ceramic bowl. Cover surface of curd with plastic wrap. Stand at room temperature for 10 minutes. Refrigerate for 4 hours or until cold and thickened.
Make Fritters: Heat oil in a saucepan over medium-high heat (see note). Sift flour, baking powder and salt.
Whisk eggs and caster sugar until just combined. Add ricotta, lemon rind and vanilla. Stir until mixture is smooth and well combined. Add flour mixture. Stir until smooth.
Drop 1 level tablespoon of dough into hot oil. Repeat 5 more times. Cook, turning occasionally, for 2 to 3 minutes or until golden all over. Drain on a tray lined with paper towel. Repeat with remaining dough. Place fritters on a serving plate. Dust with icing sugar. Serve fritters with lemon curd.