• Preheat oven to 350°F.

Prepare the Cake Pans:

• Spray 6-inch or 8-inch round pans with non-stick baking spray containing flour, such as Pam for Baking. Alternatively, line the bottoms of the pans with parchment paper and grease and flour the sides of the pan and the parchment. This recipe will make enough batter for four 6-inch layers or three thin 8-inch layers.
• Add wet cake baking strips around the outsides of the pans.
• Set aside the pans.

Make the Cake Batter:

• In a medium bowl, sift together the flour, baking powder, and salt. Set aside.
• In the bowl of stand mixer, beat the butter, oil, and sugar until fully combined.
• Add the eggs one at a time. Beat thoroughly after each egg.
• Add the vanilla extract and mix until fully incorporated.
• Using a large rubber spatula, fold in ⅓ of the dry ingredients.
• Add half of the buttermilk and fold until combined, then add another ⅓ of the dry ingredients, followed by the remaining buttermilk and ending with the rest of the dry ingredients. Small lumps of flour will remain. Take care not to overmix.

Color and Pipe the Batter:

• Remove 1 cup of batter to a bowl and tint it with black icing gel by carefully folding the gel into the batter to achieve a deep black color.
• Remove ¼ to ½ cup of cake batter to each of 3 bowls (for a 6-inch cake) or 5 bowls (for an 8-inch cake). Add one color gel to each bowl and fold into the batter carefully.
  
• Colour batter as required and put batter into piping bags.
  
• Transfer the black and colored batters to disposable piping bags, secured at the top with a rubber band. Place the remaining untinted batter into a piping bag.
• Place ½ cup of vanilla cake batter into the bottom of each prepared cake pan.
  
• Use an offset spatula to smooth the batter, covering the entire bottom of each pan.
  
• Snip off tips of the piping bags to create a small circular shaped opening. You want the opening for the black to be slightly smaller than for the colors.
• Using the black cake batter, pipe 3 circles (for 6-inch pans) or 5 circles (for 8- or 9-inch pans) on top of the untinted cake batter layer, starting about ½" from the edge of the pan, and working towards the center.
  
• Use the pink batter to pipe on top of the black outer ring of one pan and the inner ring of the other pan. This will ensure the colors are staggered when the cakes are eventually stacked. Cover the black twice with the pink.
  
• Repeat this process with the blue, piping it on the outer ring of one pan and the middle ring of the other pan. Then, repeat with the purple on the inner ring of one pan and the middle ring of the other pan. Again, you want the colored circles go around twice.
  
• Using the black cake batter, pipe a single circle on top of all of the colored circles. Then, pipe the untinted batter between all of the black circles and in the center and perimeter of the pan. This should create an even layer of cake batter.
  
• With plain vanilla batter in a piping bag, fill empty spaces between circles with batter. Then, pipe the batter on top of all of the circles to cover up the color. No need to smooth with a spatula or you risk smearing the colored batter.
  

Bake the Cakes:

• Bake the cakes for 25-30 minutes or until a toothpick inserted into the center comes out clean. This time will vary depending on the size of your cake pans and the thickness of the batter. Reduce the baking time for thin layers. You do not want to overbake this cake, or it will taste dry.
• Remove the pans from the oven, take off the cake baking strips, and allow them to cool for 10 to 15 minutes, then invert the cakes onto cooling racks to cool completely.

Assemble and Decorate the Cake:

• Once cooled, you can assemble the cake by covering a cake with buttercream and stacking the layers.
• Cover the entire cake with buttercream and decorate as desired.