Place egg whites in a large mixing bowl; let stand at room temperature for 30 minutes.
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 soft peaks form. 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 inch tube pan. Cut through batter with a knife to remove air pockets.
Bake on lowest oven rack at 350* for 35-40 minutes or until golden brown and entire top appears dry. Immediately invert pan; cool completely, about 1 hour.
Run a knife around side and center tube of pan. Remove cake; split into two horizontal layers. Place cake bottom on an ovenproof plate. Spread with lemon curd; replace cake top.
For merangue, in a small mixing 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* for 15-18 minutes or until golden brown. Refrigerate leftovers.