RAINBOW CAKE

Steps:

• Preheat the oven to 375 degrees F. Spray the bottom and sides of six 8-inch round cake pans; dust with flour and tap out excess.
• Whisk together the flour, baking powder and salt in a medium bowl; set aside.
• Beat the butter, sugar and oil in a large bowl with an electric mixer on medium-high speed until light in color and fluffy, about 5 minutes. Beat in the eggs, one at a time, then the vanilla seeds, until combined.
• Transfer the creamed mixture to a very large mixing bowl, then alternate folding in the flour mixture and the milk with a spatula, adding the flour in 3 additions and the milk in 2, starting and ending with the flour, just until smooth. Divide the batter equally among 6 mixing bowls. Using gel food colors, tint each bowl with one of the colors of the rainbow. Pour each color into its own prepared cake pan.
• Bake, rotating the pans halfway through, until a toothpick inserted in the centers comes out clean, 18 to 20 minutes. Cool the cakes in the pans on a rack for 10 minutes, then remove the cakes to the rack to cool completely.
• Level the tops of any cake layers that have domed by slicing the domes off with a serrated knife. Put some buttercream into a pastry bag fitted with a large round tip. Put the bottom cake layer onto a cake plate. Pipe around the edge of the cake layer to create a frosting "dam." Fill the center with a thin layer of jam (about 1/2 cup). Top with another cake layer and repeat until you have stacked and filled all the layers. Frost the outside of the cake with the remaining buttercream and decorate as desired.
• In the bowl of a stand mixer, combine the confectioners' sugar, butter, milk, vanilla seeds and salt. Beat on low speed for a while. When the sugar is incorporated, turn the speed up and beat until the buttercream is fluffy and smooth. If it's too wet, add a bit more confectioners' sugar. If it's too stiff, add a bit more butter. Be sure to scrape the sides of the bowl once or twice to make sure everything is combined.
• Use immediately or store in an airtight container at room temperature for up to 24 hours or in the fridge for up to 2 weeks. Rewhip cold buttercream before using.