How to Make It
Preheat oven to 350°.
To prepare cake, lightly coat 2 (8-inch) round metal cake pans with cooking spray; line bottoms of pans with wax paper. Coat wax paper with cooking spray; dust each pan with 1 teaspoon flour.
Weigh or lightly spoon 11 ounces (about 2 3/4 cups) flour into dry measuring cups; level with a knife. Combine 11 ounces flour, baking powder, baking soda, and 1/2 teaspoon salt in a medium bowl, stirring well with a whisk. Combine buttermilk and low-fat milk. Combine 1 cup sugar and butter in a large bowl; beat with mixer at medium speed until well blended. Add canola oil and egg yolks, 1 at a time, beating well after each addition. Beat in 1 teaspoon vanilla extract. Add flour mixture and milk mixture alternately to butter mixture, beginning and ending with flour mixture and beating just until combined.
Place 3 egg whites in a medium bowl; beat with a mixer at high speed until foamy, using clean, dry beaters. Gradually add remaining 2 tablespoons sugar, beating until stiff peaks form. Gently fold egg white mixture into batter. Divide batter evenly between prepared pans. Bake at 350° for 28 minutes or until a wooden pick inserted into center comes out clean. Cool in pans 10 minutes on wire racks. Loosen edges with a knife, and invert cakes onto racks. Cool completely; discard wax paper.
To prepare frosting, combine 2/3 cup sugar, 1/4 cup water, and dash of salt in a small saucepan over medium-high heat; bring to a boil, stirring just until sugar dissolves. Cook, without stirring, until a candy thermometer registers 240° (about 4 minutes). Combine cream of tartar and 3 egg whites in large bowl; beat with a mixer at high speed until foamy. Gradually add the remaining 2 tablespoons sugar, beating at high speed until medium peaks form. Gradually pour the hot sugar syrup into the egg white mixture, beating first at medium speed and then at high speed until stiff peaks form. Beat in 1/2 teaspoon vanilla extract.
To prepare filling, place lemon curd in a medium bowl. Gently fold 1/3 cup meringue into the curd. Fold an additional 2/3 cup meringue into curd mixture.
Place 1 cake layer on a plate; spread filling over top of cake, leaving a 1/4-inch border. Top with remaining cake layer. Spread the remaining frosting over sides and top of cake.