For lemon curd, in a small bowl, combine eggs and egg yolks. In a heavy saucepan, cook and stir the sugar, lemon juice and butter over medium heat until smooth. Stir a small amount of hot mixture into eggs; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir until thickened, about 2 minutes. Cool slightly. Cover and chill until thickened, about 1-1/2 hours or overnight., Preheat oven to 350°. In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add eggs, 1 at a time, beating well after each addition. Combine the ricotta cheese, buttermilk, lemon zest, vanilla and lemon juice. Combine the flour, baking powder, baking soda and salt; add to the creamed mixture alternately with the buttermilk mixture, beating well after each addition., Pour into 2 greased and floured 9-in. round baking pans. Bake until a toothpick inserted in the center comes out clean, 30-35 minutes. Cool for 10 minutes before removing from pans to wire racks to cool completely., Using a citrus zester, remove zest from lemons in long narrow strips; toss with sugar. Let stand for 30 minutes. (Save fruit for another use.) Meanwhile, to make the frosting, in a large bowl, cream butter until light and fluffy. Add the confectioners’ sugar, milk, grated lemon zest, vanilla and salt; beat until smooth., Cut each cake in half horizontally. Place 1 cake layer on a serving plate. Pipe a circle of frosting around edge of cake. Spread a third of the lemon curd inside frosting. Repeat layers twice. Top with remaining cake layer. Frost top and sides. If desired, pipe a decorative border of frosting using a star tip, and decorate sides of cake with colored sugar. Garnish with lemon zest strips. Store in the refrigerator.