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1
Heat oven to 350. Grease 3 eight inch round cake pans and line with parchment paper. Grease the parchment as well.
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2
Sift together flour, cream of tartar, and salt and set aside.
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3
In a mixer bowl, cream shortening and sugar on medium high speed, using the paddle attachment. This should take at least 5 minutes. Scrape down sides of bowl once or twice.
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4
Gradually add egg whites, a couple at a time, and beat well to combine. Add vanilla.
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5
Add flour mixture alternately with the buttermilk, that is 1/3 of the flour, 1/2 of the buttermilk, then 1/3 of flour, remaining buttermilk, then remaining flour. Mix well after each addition. Do not overbeat.
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6
In a 1/2 cup measure, combine water, vinegar, and baking soda. Fold into cake batter with a rubber spatula.
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7
Divide batter into 6 portions into small bowls. There should be about 55 ounces of batter, so divide by weight for best results.
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8
Add about 1/4 teaspoons gel color to each portion and mix well.
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9
Pour 3 of the portions into the prepared pans, one color in each pan. Bake for 16 to 18 minutes, until cakes spring back when lightly touched near center. Rearrange pans after 10 minutes, for even baking.
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10
Remove from oven and cool cakes in pan for 10 minutes, then turn out onto cooling rack lined with a tea towel, if desired.
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11
Wipe out cake pans, turn parchment paper over and grease paper again. Repeat steps 9 and 10 with remaining 3 colors of batter.
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12
Make flour frosting, using 2 cups sugar and shortening, 1/2 cup flour and 2 cups milk. See directions at flour frosting recipe on this site. http://www.food.com/recipe/flour-frosting-42282.
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13
Use a little less than 1 cup frosting between layers. Then frost sides, and finally the top. It's important to fully cover this cake for freshness and to keep the secret as a surprise.
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14
Chill for at least 2 hours before serving, for best flavor.
