FOUR-LAYER CAKE WITH LEMON CURD

Steps:

• For lemon curd:
• Combine all ingredients in heavy medium nonaluminum saucepan. Whisk over medium heat until mixture thickens and candy thermometer registers 170°F, about 8 minutes (do not boil). Pour into bowl and cool.
• For cake:
• Position rack in center of oven and preheat to 375°F. Butter and flour four 9-inch-diameter cake pans with 1 1/2-inch-high sides. Sift flour, cream of tartar, baking soda and salt into medium bowl. Beat sugar and butter in large bowl until fluffy. Add egg whites in 4 additions, beating well after each addition. Beat in vanilla. Beat dry ingredients into butter mixture alternately with milk, beginning and ending with dry ingredients.
• Divide batter among prepared pans. Bake cakes until golden and tester inserted into center comes out clean, about 20 minutes. Transfer pans to racks and cool 5 minutes. Turn cakes out onto racks; cool completely. Layers will be thin. (Lemon curd and cakes can be made 1 day ahead. Cover and chill curd. Wrap cake layers separately and store at room temperature.)
• Place 1 cake layer on platter. Spread 6 tablespoons lemon curd over. Top with second cake layer. Spread 6 tablespoons curd over. Top with third layer. Spread 6 tablespoons curd over. Top with remaining layer. (Can be made 6 hours ahead. Cover cake and remaining curd separately; chill. Bring cake to room temperature before continuing.) Whisk remaining curd in saucepan over low heat until heated through and slightly thinner. Pour atop cake, spreading evenly and allowing some to run down sides. Garnish with mint.