Preparation

1. For the cupcakes: Place rack in center of oven. Pre-heat oven to 177°C (350°F).  Line cupcake pan with paper liners.

2. In a large bowl, sift together the flour, cornstarch, baking powder and salt. In a medium bowl or large measuring cup, whisk together the milk and egg whites.

3. In the bowl of a stand mixer fitted with a paddle attachment (or a hand mixer), cream together the butter and sugar at medium speed for 2-3 minutes, or until the butter and sugar are light and creamy. While still beating at medium speed, add the vanilla extract and 1/3 of the flour mixture.  Then beat in half of the milk-egg mixture.  Beat in half of the remaining dry ingredients until well incorporated, then add the rest of the milk-egg mixture, beating until fully incorporated.  Finally, add the last of the dry ingredients and mix for an additional minute until thoroughly mixed and airy. Use a rubber spatula to fold the sprinkles into the batter.

4. Use a large scoop (approx. 45 ml/3 tablespoons) to distribute the batter to make 18-20 cupcakes. Fill each cupcake well with 2/3 of the batter. Bake for 18-24 minutes, or until a toothpick inserted into the centers comes out clean. Cool on wire rack.

5.  For the rainbow buttercream: In the bowl of a stand mixer fitted with a paddle attachment, beat the softened butter and cream cheese on medium-high speed for 5-6 minutes. With the stand mixer on low speed, add in the salt and then the powdered sugar (¼ cup at a time), and continue beating until the sugar is fully incorporated. Add in the vanilla and mix on low speed until incorporated.

6. Turn the mixer back up to medium-high speed and beat for an additional 3-4 minutes. If the buttercream is too thick, add a bit of milk or cream, one teaspoon at a time. Fit a large piping bag with your piping tip of choice. For the cupcakes in the picture, a large closed star tip was used.  Set piping bag to the side.

7. Split the buttercream into 6 separate bowls and add a drop or two of each color to each of the bowls and mix to incorporate each color (red, orange, yellow, green, blue and purple). Note: too much coloring can affect the taste of the buttercream so less is more.

8. To make the piping swirls, put a large piece of plastic wrap on the counter and add lines of the colored buttercream in the following order: red, orange, yellow, green, blue and purple.  Wrap the plastic wrap around the buttercream to make a log.  Cut off one tip of the plastic wrap log and fit it into the large piping bag with the piping tip already inserted. For the cupcakes in the picture, a large swirl was made starting in the center of each cupcake and working outwards.