Position a rack in the center of the oven and preheat to 350 (F). Generously spray three 6-inch cake pans with cooking spray and line the bottom of each pan with a parchment paper circle. Spray the parchment paper too, and set aside.

Use a tall glass to create a well in the center of dry ingredients. Pour the wet ingredients into the center of the well and whisk to combine into the dry ingredients. Pour in 1/2 cup melted butter and 1/4 cup canola oil, and whisk until just combined.

Divide the batter evenly into three bowls, about 13.35 ounces each. Tint each bowl of batter a different color by adding a drop or two of food coloring into each bowl and whisking until combined.

Pour a small amount of batter (around 4.45 ounces), one different color per pan, into the three prepared pans. Pour a small amount (around 4.45 ounces) of a second, different color into the center of each pan, on top of the first color. Repeat, making a bull's eye effect, until all the batter has been used. After the process, each cake pan should have about 13.35 ounces of batter.

With the mixer on low, gradually add all but 1 cup (4 ounces) of the confectioners' sugar, 3 tablespoons whole milk, 2 teaspoons pure vanilla extract, and a pinch of kosher salt. Once combined, increase the mixer to medium-high and beat for 3 to 5 minutes, or until the buttercream is white, fluffy, and smooth. Reduce the mixer to low and add the remaining 1 cup of confectioner's sugar as needed 1/4 cup at a time, until the desired consistency is reached. The buttercream should be soft and spreadable, but not runny.

Level the cakes and assemble into a three layer stack, placing about 1/2 cup frosting in between each layer. Cover completely with the remaining frosting and use an offset spatula to smooth the top and sides of the cake completely.