CAKE

1. Preheat oven to 350 degrees.  Line two 8-inch cake pans with a piece of waxed or parchment paper. Grease and lightly flour the paper, but not the sides of the pans.
2. To the bowl of a stand mixer, (or use a hand mixer), add the cake flour, baking soda, baking powder, salt and 1 cup of the granulated sugar.  Turn the mixer on low to thoroughly combine all of the ingredients.  On low speed, add 1/2 of the sour cream and the butter, one tablespoon at a time.  Stop and scrape down the sides and bottom of the mixing bowl.  Mixing on low speed, add the remaining sour cream,  oil, and vanilla.  Mix on medium speed until the mixture becomes light and airy, about 2 minutes.
3. In a separate bowl, using the whisk attachment, whip the egg whites and cream of tartar together on medium-high speed until they form soft peaks.  Turn the mixer to medium-low and slowly add the remaining 1/2 cup of sugar.  Turn the mixture to medium-high speed and mix until stiff shiny peaks begin to form.  Do not over beat!  Once the egg whites look stiff and shiny, stop the mixer.  If they look dry, you’ve over mixed, and the cake layers may not rise as much.
4. Using a spatula, fold the egg whites into the first mixture, one third at a time, folding only until there are slight streaks of egg white remaining.  Note:  To fold, simply run a spatula down the side of the mixing bowl, under the mixture and back up and over.  Divide the batter evenly between the two cake pans, using an offset spatula to spread evenly.  Gently tap the pans on a counter to remove air bubbles.
5.  Bake the cakes for 30 to 35 minutes until the cakes are golden brown on top and a toothpick inserted comes out with moist crumbs on it.  Ovens vary!  I suggest doing the first toothpick check at 25 minutes.
6. Cool for 15 minutes in the pans and invert the cakes onto a wire cooling rack to finish cooling.

SEVEN MINUTE FROSTING

1. Fill a medium saucepan about 1/2 full with water.  Bring it to a boil over medium heat and reduce the heat to medium-low so that it just simmering around the edges, not boiling.
2. Add the egg whites, sugar, water, and cream of tartar to a large heatproof bowl of a stand mixer.  If you don’t have a stand mixer, just make sure you use a heatproof bowl.
3. Place the bowl over the saucepan of simmering water.  Whisk the mixture frequently, for about 5 minutes, until the sugar has completely dissolved and it will also be foamy.  This takes about 5 minutes.  Check it by rubbing a tiny bit of the mixture between two fingers.  If it feels grainy, the sugar has not dissolved yet.
4. Place the bowl on the stand mixer fitted with the whisk attachment.  Beat on high speed until the frosting is very glossy and triples in volume, about 7 minutes.  Mix in the vanilla and any food coloring, if using.
5.  Spread the frosting on a cake or cupcakes as desired.
6. This frosting will hold up for 1 day at room temperature.  After day 1, this frosting will slowly begin to disintegrate and seep into the cake.  It’s best to serve cake, the day it’s made or the very next day.