Preheat oven to 350°F. Line three 8 inch round pans with parchment rounds, and grease with non-stick cooking spray.

Mix together all dry ingredients (flour, baking powder, sugar, and salt) in a stand mixer with a paddle until fully combined.

Mix chunks of room-temperature butter slowly into the dry mix, on a low speed. Continue to mix until no large chunks of butter remain, and the mixture becomes crumbly.

Pour in egg whites, and mix on low until just incorporated. Mix in the buttermilk in two installments, on a low speed.

Add in vanilla and oil, and mix at a low speed until fully incorporated. Scrape down the sides of the bowl with a spatula, then beat on medium speed for about 30 seconds. 

Divide the batter evenly between six bowls. Color each bowl a different shade (red, orange, yellow, green, blue, & purple) using gel food coloring. Mix until the batter is evenly colored.

Add spoonfuls of blue and purple batter into one cake pan. Once all the batter is in the pan, drag a small offset spatula or butter knife through the batter in horizontal and vertical lines to create a marbled pattern.

Repeat with the remaining two pans, creating a green and yellow cake layer, and a red and orange cake layer.

Bake for 34-37 minutes, or until a toothpick comes out clean. Allow the pans to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan.

Place cake layers into the freezer for 45 minutes, to accelerate the cooling process. Once the layers are fully cooled, carefully flip and remove the layers from the pans.

Use a serrated knife to level the tops of the layers.

While the cake layers bake and cool, make the vanilla buttercream frosting.

Beat the butter on a medium speed for 30 seconds with a paddle attachment, until smooth. Slowly add in the powdered sugar, 1 cup at a time. Alternate with small splashes of cream.

Once fully mixed, add in the vanilla and salt, and beat on low until the ingredients are fully incorporated, and the desired consistency is reached. 

If the frosting is too thick, add in additional cream (1 teaspoon at a time). If the frosting is too thin, add in more powdered sugar (quarter of a cup at a time).

Wait to color the remaining buttercream until the cake is assembled and frosted.

Stack and frost cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.

Add an even layer of buttercream between each cake layer.

Apply a thin coat of frosting around the the cake, to fully cover the cake layers. Smooth using a bench scraper, then chill the cake in the fridge (10 minutes) or freezer (5 minutes) until the frosting is firm to the touch.

Add a second, thicker layer of frosting to the cake, and smooth using a bench scraper.

Divide the remaining buttercream between several bowls, and add gel food coloring to create the same colors as the cake layers. If desired, you can create different shades of some of the colors.

Fit several small piping bags with french frosting tips, and fill with the different colored frostings.

Pipe frosting around the sides of the cake, to cover it in a rainbow pattern.