Infuse the butter for the cake: Rub 6 rose geranium leaves to release the oils. Wrap the leaves around the butter. Wrap in plastic wrap and refrigerate overnight.
Infuse the sugar for the frosting: Rub 4 rose geranium leaves, then combine with 1 1/2 cups sugar in an airtight container and let stand at room temperature overnight.
Make the cake: Preheat the oven to 350 degrees F. Butter and flour a 9-by-13-inch baking pan. Rub the remaining 10 to 12 leaves and arrange in the pan. Whisk the flour, baking powder and salt in a bowl. In a separate bowl, mix the milk, 3/4 cup water and the vanilla. Unwrap the butter and remove the leaves. Beat the butter and 1 3/4 cups sugar in a large bowl with a mixer on medium speed until fluffy. Beat in the egg whites, two at a time. Add the flour mixture in three parts, alternating with the milk mixture, beginning and ending with flour. Pour the batter into the pan; bake until golden, 30 to 35 minutes. Cool 10 minutes; invert the cake onto a rack and remove the leaves.
Make the frosting: Remove the leaves from the sugar. Put the sugar, egg whites, cream of tartar, salt and 1/3 cup cold water in a large heatproof bowl and beat with a mixer, 1 minute. Place the bowl over a pot of barely simmering water. Beat until the mixture holds stiff peaks, about 7 minutes. Remove from the heat, add the food coloring and beat until spreadable, about 2 minutes. Spread the frosting over the cake.
Photograph by Kana Okada
Courtesy of Anne Burrell for Food Network Magazine