Preheat oven to 350. Spray a 15 by 10 by 1 inch je)y roll pan with bakers spray. Line pan with parchment paper and spray paper with parchment spray
Have egg whites in one large bowl and egg yolks in another large bowl and have them both at room temperature
Whisk flour, baking powder and salt in a bowl and set aside
Beat egg yolks and sour cream until light in color and increased in volume. Beat in 1 cup of the sugar and vanilla until well blended
Stir in the flour mixture, make sure it's well incoorated but don't overmix
With clean and dry beaters beat egg whites and cream of tarter until soft peaks form, Slowly add the remaining sugar and beat until glossy peaks form
Fold 1/4 of egg white mixture into egg yolk mixture then fold in remaining in 3 more additions
Carfully seperate batter into 3 bowls, trying not to deflate to much
Add drops of food color, blue to one bowl. Red to the second bowl and yellow to the third, fold just until color is uniform.
Spread each layer of batter in stripes in prepared pan as shown belo, smooth tops.
Bake 16 to 18 minutes until cake springs back when touched and is no longer sticky feeling to touch. Cool 5 minutes in pan on rack then run a small knife around the edges to unstuck any ends of cake
After cake cools 5 minutes Invert onto confectioner's sugar dusted kitchen towel. Carefully peel off parchment paper
Roll cake up in towel and cool on rack
Unroll cooled cake and spread softened sherbet over surface, leaving some space at edges. Roll up using towel to help keep tight and cover and frezze at least 6 hours before frosting
Frost cake with White Chocolate Ganache Frosting, recipe attached below, then cover with sprinkles and re frezze until hard about 2 hours
https://cookpad.com/us/recipes/359637-white-chocolate-ganache-whipped-frosting-filling
