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1
Combine all ingredients in heavy medium nonaluminum saucepan.
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2
Whisk over medium heat until mixture thickens and candy thermometer registers 170F, about 8 minutes (do not boil).
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3
Pour into bowl and cool.
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4
Position rack in center of oven and preheat to 375F.
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5
Butter and flour four 9-inch-diameter cake pans with 1 1/2-inch-high sides.
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6
Sift flour, cream of tartar, baking soda and salt into medium bowl.
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7
Beat sugar and butter in large bowl until fluffy.
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8
Add egg whites in 4 additions, beating well after each addition.
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9
Beat in vanilla.
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10
Beat dry ingredients into butter mixture alternately with milk, beginning and ending with dry ingredients.
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11
Divide batter among prepared pans.
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12
Bake cakes until golden and tester inserted into center comes out clean, about 20 minutes.
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13
Transfer pans to racks and cool 5 minutes.
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14
Turn cakes out onto racks; cool completely.
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15
Layers will be thin.
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16
(Lemon curd and cakes can be made 1 day ahead.
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17
Cover and chill curd.
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18
Wrap cake layers separately and store at room temperature.)
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19
Place 1 cake layer on platter.
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20
Spread 6 tablespoons lemon curd over.
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21
Top with second cake layer.
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22
Spread 6 tablespoons curd over.
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23
Top with third layer.
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24
Spread 6 tablespoons curd over.
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25
Top with remaining layer.
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26
(Can be made 6 hours ahead.
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27
Cover cake and remaining curd separately; chill.
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28
Bring cake to room temperature before continuing.)
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29
Whisk remaining curd in saucepan over low heat until heated through and slightly thinner.
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30
Pour atop cake, spreading evenly and allowing some to run down sides.
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31
Garnish with mint.
