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1
Preheat an oven to 375 degrees F (190 C).
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2
Sift the flour onto a plate, then stir in the lemon zest and set aside.
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3
Put the egg whites, salt and cream of tartar into a large bowl and beat until stiff but moist-looking peaks form.
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4
Gradually add in the superfine sugar, a tablespoon at a time, and continue beating for 1 more minute.
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5
Gently fold in the flour mixture using a metal spoon and a swirling figure-of-eight motion.
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6
Pour into an 8-inch deep or 9-inch shallow nonstick angel food cake pan (there is no need to grease it first).
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7
Bake until the cake is well risen and golden and the top springs back when pressed with a fingertip, 25 to 30 minutes.
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8
Turn the cake, still in its pan, upside down onto a cooling rack and let cool.
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9
As it cools, it will fall out of the pan.
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10
To Make the Frosting:.
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11
In a bowl, stir together the lemon curd and sour cream and spread over the top of the cooled cake.
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12
Sprinkle with crystallized rose petals or flowers.
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13
Note: To crystallize flowers, buy a sachet of dried egg white and mix it as instructed.
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14
Let it cool, then brush the mixture over rose petals or viola, pansy or herb flowers.
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15
Dust lightly with a little superfine sugar, let dry for 30 minutes, then use to decorate the cake just before serving.
