Preheat your oven to 325°F (163°C) and prepare a half-sheet pan lined with parchment paper.

In a large mixing bowl, combine sour cream, buttermilk, vanilla extract, egg whites, and oil. Beat until smooth.

Sift in the white cake mix and stir just to combine.

Divide the batter into six bowls and color each with food coloring gel.

Pipe the colored batter diagonally across the prepared pan in rainbow order.

Bake for about 17 minutes until the cake springs back lightly to the touch.

Let the cake cool for 5 minutes, then roll it tightly from the shorter side while still on the parchment paper.

Allow the rolled cake to cool completely on a wire rack for about 40 minutes.

Beat together butter and salt, then gradually add powdered sugar, vanilla, and cream to make frosting.

Unroll the cooled cake and spread buttercream evenly, leaving a margin.

Re-roll the cake tightly, peeling back the parchment as you go.

Wrap in plastic wrap and chill in the freezer for at least 1 hour before slicing.