Place 12 egg whites in a large bowl. Sift 1/2 cup sugar and flour together twice; set aside. , Add cream of tartar, vanilla and salt to egg whites; beat on medium speed until foamy. Gradually beat in remaining sugar, 2 tablespoons at a time, on high until stiff glossy peaks form and sugar is dissolved. Gradually fold in flour mixture, about 1/2 cup at a time. , Gently spoon batter into an ungreased 10-in. tube pan. Cut through batter with a knife to remove air pockets. Bake on lowest oven rack at 350° until golden brown and entire top appears dry, 35-40 minutes. Immediately invert pan; cool completely, about 1 hour., Run a knife around side and center tube of pan. Remove cake; split into 2 horizontal layers. Place cake bottom on a parchment-lined baking sheet. Spread with lemon curd; replace cake top. , For meringue, in a small bowl, beat egg whites and cream of tartar on medium until soft peaks form. Gradually beat in sugar, 1 tablespoon at a time, on high until stiff glossy peaks form and sugar is dissolved. Spread over top and sides of cake., Bake at 350° until golden brown, 15-18 minutes. Transfer to a serving plate. Refrigerate leftovers.