Preheat the oven to 350 degrees F. Spray the bottom and sides of four 6-inch round cake pans with nonstick spray, line the bottom with parchment paper, and spray again with the nonstick spray. Set aside.

In a stand mixer fitted with the paddle attachment, combine the butter, granulated sugar, brown sugar, and green food gel. Beat the ingredients on medium-high speed for about 5 minutes, until the mixture is light in color and slightly increased in volume.

Lower the speed and add the eggs, one at a time, mixing until each egg is incorporated before adding the next.

In a separate bowl, sift the flour and cocoa powder. Sprinkle the salt on top of the dry ingredients and whisk until combined.

With the stand mixer on low speed, alternately add the dry ingredients and the buttermilk into the butter mixture, beginning and ending with dry ingredients. Mix until just incorporated. Fold in the sour cream.

In a small bowl, combine the vinegar and baking soda until it fizzes and then add to the batter. Whisk lightly until blended.

Divide the batter evenly between the prepared pans (about 13-14 ounces of batter in each pan).

Bake for 23-25 minutes. 

Let pans cool on wire rack for about 10-15 minutes before inverting the cakes onto racks. Let cool completely.

Level cakes. Use the cakes right away or wrap in plastic wrap and freeze for a few days. 

In a stand mixer fitted with a paddle attachment, cream the butter on medium-high speed for about two minutes.

With the mixer on low speed, slowly add the sifted powdered sugar until incorporated. Scrape down the sides of the bowl and continue to mix for another minute.

Add the vanilla, salt and heavy cream (one tablespoon at a time). Once the cream is incorporated, increase the speed to medium-high and beat the frosting for five minutes. It should get lighter in texture and color.

Divide the frosting into six bowls and add food coloring, if desired.

Use a spoon to mix the frosting by hand, which will push out the air pockets created by the electric mixer.

Once the cake layers are completely cooled and leveled, use a 4" cake ring or cookie cutter to cut the center of two cake layers out.

Place the first cake layer (not one that has the center out) bottom-side down, in the center of the cake board. Using an offset palette knife, spread about 1/2 cup frosting evenly across the cake layer. 

Place the second cake (one without a center) layer on top of the frosting. Frost the top of the second cake and add the third cake layer (the second cake without a center) on top of the frosting. Fill the center hole with gold coins and top with the final cake layer (which should be the second cake layer that wasn't cut out). 

Crumb coat the entire cake with a thin layer of frosting and freeze for about 10 to 15 minutes to set.

Once the crumb coat is set, continue to frost the cake with the remaining frosting.

If you're doing the rainbow effect, place each color in it's own piping bag. Pipe two stripes of each color around the entire cake, starting with red at the top and finishing with purple at the bottom. Use a cake scraper to smooth out the colors and blend.