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1
Combine all ingredients in large mixer bowl, then beat at medium speed for 2 minutes. Pour into greased and floured 9 x 13 inch pan. Bake for 30-35 minutes at 350 degrees. Cool and frost.
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2
In a mixing bowl, using an electric mixer with a paddle attachment, cream the butter and shortening together for 1 to 2 minutes, until they are light in color and texture but not too soft. Scrape this mixture into a different bowl and set it aside to use later. Clean the mixing bowl to use for the egg whites.
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3
Combine the sugar, egg whites, corn syrup, and salt in the top pan of a double boiler. Heat over simmering water, stirring frequently, until the sugar granules have dissolved and the temperature on a food thermometer registers 140u00b0F. Transfer the whites to the clean mixing bowl. Using an electric mixer with a whip attachment, whip the whites on medium to medium-high speed until the mixing bowl feels just cool to the touch, 10 to 12 minutes. The mixture should be white and fluffy and very thick.
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4
Add the butter mixture and the vanilla to the egg white mixture and whip again, on medium speed, until the frosting has a smooth, creamy, spreadable texture, almost like stiff whipped cream, 1 to 2 minutes.
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5
The frosting can be used immediately or stored in an airtight container at room temperature, but it should be used within 3 days. You may have to stir it briskly to re-fluff it if it's been sitting for a long time.
