Preheat oven to 350°F (175°C). Grease and flour two 8-inch (20 cm) round cake pans and line bottoms with parchment paper.
Cool cakes completely in pans on wire racks. When cakes are no longer warm, cover the pans tightly with plastic wrap or aluminum foil and chill for at least 2 hours.
In a medium-sized saucepan, heat and stir the cream, butter, sugar and salt over low heat until mixture nearly comes to a boil. Do not allow it to boil. Remove from heat and stir in chocolate until completely smooth. Stir in vanilla.
Allow mixture to cool to room temperature (about 2 hours), until it reaches a spreadable consistency, stirring occasionally.
Beat the butter and salt in a mixing bowl on medium speed for 3-5 minutes until the butter appears nearly white. Add the sifted powdered sugar and vanilla and mix on low speed until blended. Scrape bottom and sides of mixing bowl with a rubber spatula and mix again briefly. Add the heavy cream and mix on medium speed until the frosting is creamy.
Divide the frosting into five bowls (about 130 g each) and color each frosting a different color of your choice. Begin coloring with minimal amounts of food coloring. Transfer frosting to five plastic food storage bags.
Turn chilled cakes out of pans and remove parchment paper. Slice each cake horizontally into three layers. Ideally, you would use a cake leveler to cut the layers but a long, serrated bread knife works as well. Don’t worry if the layers aren’t perfect, the cake will still be beautiful.
Start by frosting the sides of the cake. Holding your palette knife parallel to the cake, apply plenty of frosting to the sides, filling in any holes and uneven spots with the frosting. Smooth out the frosting and remove excess by scraping the sides of the cake with palette knife or cake scraper (or a ruler). Repeat until the sides are nicely covered. Then frost the top of the cake and scrape smooth.
Special equipment: 2 round cake pans 8 inches (20 cm) in diameter, cake leveler, food coloring (preferably gel or paste), colored sprinkles, palette knife (straight or offset spatula) or cake scraper (or a ruler), Lazy Susan (turntable).
Chocolate Rainbow Cake https://livingoncookies.com/chocolate-rainbow-cake/