Place a ½ sheet pan in the freezer while you make the batter. Preheat the oven to 350 °F.
Combine 2 oz unsalted butter, 2 oz sour cream and 1 teaspoon vanilla extract in a mixing bowl, set aside. In another bowl, whisk ¼ teaspoon table salt into 4 ½ oz cake flour, set aside.
Whip 8 large egg whites to soft peak. Slowly add 12 oz granulated sugar and whip to full peak. With the mixer running on low, add 4 oz water and re-whip to full peak.
Whisk a cup of the egg whites into the butter/sour cream mixture, then whisk that back into the egg whites. Sift the flour over the egg whites and fold into the whites.
Remove ½ cup of the batter to a separate bowl. Add several drops of the red gel color to that ½ cup of batter to make a bright red color. Stir about ⅓ of the remaining white batter into the red base.
Put the red batter into a large piping bag fitted with a plain tip or a disposable piping bag. Remove the cold pan from the freezer and line with a sheet of parchment paper. Pipe diagonal lines of red batter evenly spaced in the pan. Set the pan into the freezer for 10-15 minutes until the surface of the batter is firm.
Pour the white batter over the stripes and use an offset spatula to spread to an even layer. Bake the cake about 15 minutes until the center of the cake springs back when pressed.
Let the cake cool 5 minutes in the pan then flip out onto a clean sheet of parchment paper. Peel the parchment from the bottom of the cake then flip the cake back over. Roll the cake in the parchment and let it cool while you prepare the filling.