Filling: Put Filling ingredients in a medium saucepan and stir to mix. Bring to a boil, reduce heat and simmer, stirring occasionally, until thick and nearly smooth. Let cool, stirring occasionally. Place waxed paper directly on surface (to keep a skin from forming) and refrigerate until cold or up to 2 days.
Heat oven to 350°F. Line bottom of an 8-inch springform pan with waxed paper. Grease paper. Wrap outside of pan with foil, molding it tightly around pan to keep batter from seeping out.
Cake: Put flour, baking powder and salt in a medium bowl and whisk until blended. In a small bowl, stir milk, melted butter, lemon peel and juice and vanilla until blended (mixture may look curdled).
In a large bowl with an electric mixer, beat egg yolks, whole eggs and sugar on high speed 7 minutes or until at least doubled in volume, thick, fluffy and pale.
Reduce mixer to medium speed and add flour mixture in 3 additions alternately with milk mixture, beginning and ending with flour mixture, until well blended. Scrape into prepared pan. Smooth top.
Bake 55 to 60 minutes or until a wooden pick inserted in center of cake comes out clean. Remove pan to a wire rack to cool 20 minutes. Run a knife around inside edges of pan. Remove sides of pan. While still warm, loosen cake from pan bottom with a knife and remove waxed paper. Place cake on rack to cool completely.
Whipped Cream Frosting: Put cream and sugar in a large bowl. Beat with electric mixer on high speed until stiff peaks form when beaters are lifted. Beat in vanilla extract just until blended.
To assemble: Trim “hump” off top of cake with a long, serrated knife. Cut cake into 3 even layers. Put bottom layer on a serving plate. Spread half the filling on top. Top with another cake layer and remaining filling. Top with remaining cake layer. Carefully spread frosting on sides and top of cake.
Refrigerate uncovered up to 4 hours. Decorate with sugared flowers or fanned strawberries up to 2 hours before serving.
Planning Tip: The baked, undecorated cake can be stored, well wrapped, at room temperature up to 1 day or frozen up to 1 month. The filling can be made up to 2 days ahead, but the frosting is best made no more than 4 hours before serving. Decorate with the sugared flowers or fanned strawberries no more than 2 hours before serving.
Strawberry Fans: Hold strawberry on a cutting surface and make parallel cuts from tip of berry to as close to the stem as possible. Press berry gently to fan slices.
Sugaring Flowers: Mix meringue powder or Just Whites powdered egg whites following package directions for 2 egg whites. With fingers, gently dab mixture evenly on both sides of flowers. (Don’t dip flowers or coating will be too thick.)
Sprinkle superfine granulated sugar over both sides of flowers until evenly coated. Place on a wire rack to dry, at least 8 hours.