Preheat oven to 350°F. Line three 6 inch 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 sour cream 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 three bowls. Color one yellow with gel food coloring, and one orange with gel food coloring. Leave the third uncolored.

Pour each color of batter into one of the prepared cake pans.

Bake for 32-34 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 30 minutes, to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans and remove the layers.

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 whisk 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 marshmallow fluff, vanilla and salt, and beat on a medium speed 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).

Color 1 1/2 cups of frosting yellow with gel food coloring, and place in a piping bag with no frosting tip. Color 1 cup of frosting orange with gel food coloring, and place in a separate piping bag with no frosting tip. Put 1 cup of frosting into another piping bag.

Stack and frost cake layers on a 6-inch greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.

Add an even layer of marshmallow buttercream between each cake layer.

Spread 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, using the frosting in the piping bags. Cover the bottom third of the cake with yellow buttercream. Pipe orange frosting around the middle third of the cake, and use white buttercream to cover the top third of the cake. Do not add more frosting to the top of the cake.

Smooth with a bench scraper.

Place the second greaseproof cake board on top of the cake. Add sanding sugar into a baking pan, pouring each color in the same order and width as the frosting on the side of the cake.

Carefully lift the cake sideways, and roll in the sanding sugar.

Place the cake back on your cake stand. Remove the top cake board, and cover the top of the cake with white sanding sugar.

Use a coupler to fit each frosting bag with a wilton 1M frosting tip, and pipe buttercream swirls on top of the cake.