Prepare six 8-inch round cake pans with nonstick spray and line each with a round of parchment paper. Set aside. NOTE: If you don't have six pans, prepare as many as you have for baking your first round of cakes. Allow them to cool completely before preparing them for the second round of baking.
Combine 1 cup (240 ml) of the buttermilk and all of the canola or vegetable together in a measuring cup. Set aside. In another measuring cup, combine the rest of the buttermilk (1½ cups or 360 ml) with the egg whites and vanilla. Whisk well to break up the egg whites and set aside.
In the bowl of a stand mixer (or a large bowl with a handheld mixer), measure out the flour, baking powder, baking soda, salt, and sugar. Using the paddle attachment, quickly mix these together and then add the softened butter and mix on low until it resembles coarse sand. NOTE: I pulsed this mixture initially because there's a chance some of your flour will fly out. You can also use the shield of your Kitchenaid.
Add in the buttermilk and oil mixture and mix on low just to get the dry ingredients moistened. Then increase the speed to medium and let it mix for 2 minutes. Don't skip this step or your cake could collapse.
Next add the egg white/buttermilk/vanilla mixture in three additions, allowing it to mix for 15 seconds between each addition. Scrape the sides and bottom of the bowl.
Divide the batter evenly into six small bowls. Each should weigh 15 ounces, or 425 grams.
Color the cake batter as follows: ½ tsp of pink for the pink layer, ¼ tsp yellow plus ¼ tsp orange for orange layer, ½ tsp yellow for yellow layer, ¼ tsp yellow plus ½ tsp green for green layer, ½ tsp blue for blue layer, ¼ tsp pink and ½ tsp purple for the purple layer.
Bake the cakes for 24-28 minutes, or until the cake springs back when lightly touched, or a toothpick inserted into the center comes out clean. Don't under bake the layers as they will collapse in the middle. Immediately tap the cake layers on the counter, as soon as they come out of the oven, to release the steam. This helps make even layers. Allow the cakes to cool for 10 minutes in the pans and then flip them out onto wire racks to cool completely. If you have to do a second bake (due to not having enough pans), allow the pans to cool and prepare them again (you should be able to reuse the parchment paper though).
Freeze the cake layers for 30 minutes before filling and stacking with the buttercream. Another option is to wrap each in plastic wrap and put them in the fridge overnight to make this cake a 2-day project.
Fill and frost the cake layers with buttercream, refrigerating in between crumb coat and final coat. Refrigerate the cake for at least a half an hour before applying the drip.
Apply the drip and return the cake to the fridge for at least 15 minutes before painting the drip with gold edible paint. Allow the paint to dry before finishing the cake with rainbow frosting swirls and gold coins.
For the Swiss Meringue Buttercream:
Place the egg whites and sugar in the bowl of a stand mixer and set this over a small saucepan filled with one inch of barely simmering water.
Whisk continuously until the mixture reaches 160° F, or a little rubbed between your fingers is smooth without any sugar crystals present.
Attach the bowl to your stand mixer fitted with the whisk attachment and whisk on medium high until the meringue is stiff and the bowl is cool to the touch. This will take anywhere from 5-10 minutes, longer if you don't have a stand mixer.
With the mixer still running on medium high, add the butter, one chunk at a time. The mixture may look curdled at some point while adding the butter or after. This is totally normally. It will become smooth so just keep whisking.
Switch to the paddle attachment and add the vanilla extract. Turn the mixer on to low and continue to blend just to knock out any large air bubbles, which makes it easier to smooth the frosting onto the top and sides of the cake.
After filling and frosting the cake, divide the rest of the frosting into six small bowls and color using the same color guide for the cake above, reducing the amounts to one quarter (where it calls for ½ tsp, use ⅛ and ¼ tsp, use just a few drops). Add each color to small decorating bags or sandwich baggies. Cut a half-inch off the corner of the baggies and pipe each color side by side into a stripe on a piece of plastic wrap. Fold it over itself to create a "roll" and twist the ends. Cut off one end and insert the frosting roll into a large piping bag fitted with a large open star tip (1M), cut side first (plastic wrap and all). Pipe frosting swirls onto cake as desired.
For the White Chocolate Ganache Drip:
Place chopped white chocolate into a small bowl and microwave 20 second intervals until almost melted. Microwave heavy cream in another small bowl or measuring cup until just under boiling, 20-30 seconds.
Pour hot cream over almost melted white chocolate and stir to combine. There should be no lumps of chocolate remaining. If there are, microwave at 20 second intervals and stir until completely smooth. Allow the ganache to come to barely warm (body temperature) before adding to a decorating bag. Snip off the corner of the bag and drip the ganache around the top of the cake at the edges using varying pressure to obtain different lengths of drips.