Cake Batter:

Preheat oven to 350° F / 176° C. Generously spray a quarter sheet rimmed baking tray with nonstick spray, line with parchment, then spray with more nonstick spray.

Spread a clean tea towel across a section of your work surface. Dust generously with confectioner’s sugar.

With a kitchen scale, weigh 6 portions of self rising flour into individual bowls. Each portion should weigh 28 grams (about 1 ounce).

In a large mixing bowl or the bowl of a stand mixer, whip eggs and granulated sugar on high speed until pale, fluffy, and voluminous, about 3-4 minutes. Mix in the vanilla bean paste.

With a kitchen scale, weigh batter into 6 small bowls. Each portion should weigh approximately 60 grams (about 2 ounces).

With a fine mesh sieve, sift a self-rising flour portion into each batter bowl.

Add 2-3 drops of gel food coloring to each bowl to create red, orange, yellow, green, blue, and purple batters.

With a rubber spatula, gently fold the flour and food coloring into the eggs until just barely combined, deflating the mixture as little as possible.  Do not overmix.

Scoop each color into a separate disposable piping bag.

Clip the tip of each piping bag with kitchen scissors.

Pipe batter diagonally across prepared tray, alternating colors to form the classic rainbow pattern. (For tips and detailed instructions on piping the batter, see post above.) Depending on the tray you’ve chosen you might have a small amount of batter left over, but try to use most of it or the cake will be too thin.

Bake in preheated oven for 18-20 minutes or until cake sets and feels dry to the touch.

Run a sharp knife around the edge of the cake. Immediately flip cake onto prepared tea towel. Tap bottom of tray to release cake, then peel away the parchment.

Dust top of cake with more confectioner’s sugar.

Tightly roll the hot cake in the towel, bringing the two shorter sides together.

Set rolled cake aside and allow to cool for about 1 hour, or until completely cool to the touch.

Buttercream:

In a large mixing bowl or the bowl of a stand mixer, mix the butter on high speed until light and fluffy.

Add the confectioner’s sugar in small increments, alternating with the milk and mixing well after each addition. Use a rubber spatula to scrape down the sides of the bowl as needed, until you’ve fully incorporated all of the sugar. Add the cake batter flavoring and mix until creamy, about 3-5 minutes. If necessary, add more confectioner’s sugar or whole milk to adjust the consistency. Frosting should feel smooth and easily spreadable, but not runny or thin.

With a flexible rubber spatula, fold in the rainbow nonpareils.

Assembly:

Carefully unroll the cooled cake.

With a small stepped palette knife or flexible rubber spatula, spread the buttercream evenly across the surface.

With your hands, tightly roll the cake around the buttercream, forming a log shape.

Tightly wrap log in 2-3 layers of clingfilm/plastic wrap. Place roll on a small tray and refrigerate at least 8 hours, or overnight.

To serve, carefully peel away the plastic wrap. With a serrated knife or a sharp chef’s knife, slice log into 9 slices.

Store leftover rainbow cake in the refrigerator for up to 3 days.