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1
To make the herb-oil mixture, put a fourth of the parsley and mint in a strong blender or food processor, and blend it on low speed. Use a blender stick to help crush the herbs while the blade is spinning (or stop the machine from time to time to push the herbs back down toward the blade). Slowly increase the speed to medium (or a steady puree, in a food processor) and continue adding the rest of the herbs until you have added all of them.
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2
In a steady stream, add half of the olive oil. Mix on medium-low speed (or pulsing, if using a food processor) until all is combined. Add the remaining olive oil and blend for no longer than 10 seconds. The mixture will look loose and stringy. Scrape out the blender to get all of the parsley mixture, transfer it to a bowl, and refrigerate until ready to use.
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3
In a bowl, combine the flour, cornstarch, salt, and baking powder and set aside.
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4
In a stand mixer fitted with the paddle attachment, whip the eggs for about 30 seconds. Add the sugar and mix on high speed until the mixture is very thick and turns a pale yellow color, about 3 minutes. Turn the mixer speed down to low and add the herb-oil mixture.
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5
With the machine still running, add the flour mixture and mix until just combined. Do not over mix. Pour the batter into a container and refrigerate it for at least 6 and up to 24 hours (the cake will turn out much greener than it would if you baked it right away).
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6
When you're ready to bake, preheat the oven to 340u00b0F and lightly oil a sheet pan -- ideally a 13- x 18-inch for a thin cake but 11 3/4- x 16 1/2-inch will work with a slightly longer baking time (at Food52, we used a 10- x 15-inch jelly roll pan). Line the bottom of the pan with parchment paper and lightly oil the paper. Pour the batter into the sheet pan and smooth out the top with a spatula.
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7
Bake for 12 to 18 minutes, rotating the cake halfway through. If the top begins to brown before the inside of the cake is done, turn the heat down to 330u00b0 and let it cook a couple of minutes longer. When a cake tester inserted in the center of the cake comes out clean, it's done. Let it cool in the pan.
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8
To serve, tear serving-size squares of cake into a few larger pieces and divide them among individual plates. If desired, serve with vanilla ice cream and lemon zest. Alternately, eat warm with butter for breakfast.
