Lemon and ricotta go together perfectly in this moist and delicious cake.
Wrap ricotta tightly in fine cheesecloth; place in a colander over a bowl. Place a heavy bowl or cans on top; let drain, refrigerated, for 3 hours or overnight.
Heat oven to 350 degrees. Coat a 6-inch springform pan with melted butter and 1 tablespoon sugar; set aside. Place drained ricotta, 1/4 cup sugar, flour, egg yolk, vanilla, salt, lemon zest, and lemon juice in the bowl of a food processor; blend until smooth. Transfer to a large mixing bowl, and set aside.
Bring a saucepan filled with an inch of water to a simmer; reduce heat to low. Combine egg whites and remaining 1/4 cup sugar in the heatproof bowl of an electric mixer. Place over the pan of simmering water; stir until egg whites are warm and sugar is dissolved, 2 to 3 minutes. Remove from heat; whisk on medium speed until frothy. Gradually increase speed, whisking until stiff peaks form, about 15 minutes.
Stir one-third of the egg-white mixture into the ricotta mixture; gently fold in remaining egg whites. Pour into prepared pan. Bake 40 to 45 minutes until lightly browned, firm to the touch, and center is set. Transfer from oven to a cooling rack; after 15 minutes, remove ring. Serve.
To make a chocolate-ricotta cake, omit the lemon zest and juice, and add 6 tablespoons cocoa powder and 1/4 teaspoon ground cardamom.