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1
For the frosting: Combine the egg whites, sugar, salt and vanilla extract in a heatproof bowl and set over (not in) a pan of simmering water.
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2
Stir continuously over heat until the mixture is warm and the sugar is completely dissolved, 1 to 2 minutes.
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3
Remove the bowl from the heat. Using an electric mixer, beat on high until the mixture is entirely cooled, glossy and stiff, about 7 minutes.
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4
Blend in the lemon zest until smooth - once smooth, take care not to overbeat or the mixture will get lumpy. Let frosting set and cool in the refrigerator (at least 30 minutes), while making the cupcakes.
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5
For the cupcakes: Whisk together the flour, almonds and baking powder. In another bowl, beat the eggs, sugar, salt and vanilla with a mixer until thick and light-colored, about 4 minutes.
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6
Beat in the zucchini on low speed until fully incorporated.
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7
Add the dry ingredients and beat on low speed until fully incorporated, scraping down the bowl once with a spatula during beating. Use a 1/3-cup measure to spoon into the muffin cups.
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8
Bake at 350 until a toothpick inserted into the center comes out clean, about 20 to 25 minutes, turning the pan midway through baking. Transfer cupcakes out onto a wire rack and cool completely before icing with the chilled frosting.
