Preheat oven to 350°F. Line six, 6-inch pans with parchment rounds, and grease with a light spray of non-stick cooking spray.

Mix together all dry ingredients (flour, baking powder, sugar, and salt) in a stand mixer with a paddle with a few gentle pulses.

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 low speed for about 15 seconds. 

Divide the batter evenly between six bowls (about 300 grams of batter per bowl). Create red, orange, yellow, green, blue, and purple batter using a couple drops of gel food coloring. Mix by hand with a spatula until the batter is evenly colored.

Pour each color into one of the prepared cake pans.

Bake for 30-32 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 the pans and remove the layers from the pans.

Use a serrated knife to level the top of each layer.

Heat heavy cream in a heat-proof bowl in the microwave for 45 seconds, until it's just beginning to bubble. Gently pour white chocolate chips into the heavy cream, making sure they are fully covered with cream. Allow mixture to sit for 1 minute.

Stir slowly until the cream and white chocolate are combined. Some bits of chocolate may not be fully melted yet. Heat the mixture again for 20 seconds, and stir. Repeat as necessary until all the chocolate bits are fully melted and incorporated.

Divide the ganache evenly between six small bowls. Use gel food coloring to make red, orange, yellow, green, blue, and purple white chocolate ganache. Allow to cool until desired viscosity is reached (barely warm to the touch).

Either pour into plastic squirt bottles (if you have some on hand), or place plastic wrap over the bowls to prevent the ganache from forming a skin.

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).

Cover with plastic wrap to prevent crusting, and set aside.

Stack and frost the rainbow cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board. Start with the red layer on the bottom of the cake, then follow the color spectrum (ROYGBV) to arrange the remaining cake layers.

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. Place the cake in the freezer to chill for 10-15 minutes, or in the fridge for 30 minutes.

Carefully divide the top of the cake into six equal sections (almost like slices of cake).

Add one color of white chocolate ganache drips to each section. Use either a plastic squirt bottle and a spoon to add the drips to the cake.

Top each section with matching lollipops and candy.