Confetti Cakes:

Preheat oven to 375° F / 190° C. Line two baking trays with silicone mats, and set aside until ready to use.

In a large bowl, sieve flour, baking powder, baking soda, and salt.

In a separate mixing bowl or the bowl of a stand mixer, mix butter on high speed until light and fluffy. Mix in the granulated sugar, followed by the egg and vanilla extract, and continue mixing on high speed until well combined.

Add the dry ingredient mixture to the batter, followed by the milk. Mix until just combined.

Gently stir in the rainbow sprinkles, making sure they are evenly distributed throughout the batter.

Transfer batter to a piping bag with 1/2-inch round tip. Pipe 1 – 1 1/4-inch rounds onto prepared trays, allowing space for the cakes to spread. With your fingertips, pat the tops of each cake to create a smooth, evenly domed surface.

Top with additional sprinkles.

Transfer tray to refrigerator to chill for 20 minutes.

Once chilled, bake in preheated oven for 8-9 minutes, or until the edges of the cakes are very lightly golden brown. Cool on trays.

While cakes are cooling, prepare the rainbow buttercream.

Rainbow 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, mixing well after each addition and alternating with the milk.

Add the cake batter flavoring and salt. Mix on high speed until until smooth and creamy, about 3-5 minutes. If necessary, add more confectioner’s sugar or milk to adjust the consistency. The buttercream should be thick and creamy, but still soft enough to pipe.

Portion buttercream into six individual bowls.

Add 1-2 drops of gel food coloring to each bowl.

Stir to create six differently-colored buttercreams. You should have: red, orange, yellow, green, blue, and purple.

Transfer each color to its own piping bag.

On a piece of clingfilm/plastic wrap, pipe a stripe of each color, mimicking the color pattern of the rainbow. Use all of the buttercream.

Gently roll the clingfilm around the buttercream, forming a tube.

Trim excess clingfilm from the ends of the tube, then place the tube inside a larger piping bag fitted with a star piping tip.

Assembly:

Divide the cakes into pairs, choosing tops and bottoms that are as close as possible in size.

Pipe a generous amount of buttercream on the flat side of the ‘bottom’ cakes.

Top each with the second cake to create a sandwich.

Store whoopie pies in an airtight container, layered between sheets of parchment, for up to five days.