Preheat the oven to 350°F (177°C) for convection ovens or 325°F (163°C) for conventional ovens. Line 13 muffin tins with cupcake liners, preferably using a light-colored pan. In a medium bowl, whisk together the cake flour, baking soda, baking powder, salt, and sugar: 1½ cups (172g) cake flour, ¼ tsp baking soda, ½ tsp baking powder, ¼ tsp salt, and 1 cup (200g) granulated sugar. Add the canola oil and unsalted butter to the dry ingredients and beat until combined. Next, mix in the egg whites and both vanilla extracts until fully incorporated, then add the whole milk and sour cream. Gently fold in ⅓ cup of pastel rainbow sequin sprinkles. Mix the batter just until smooth, then fill each cupcake liner about two-thirds to three-quarters full. Bake for 16–18 minutes, or until the cupcake springs back when lightly pressed or a toothpick inserted comes out clean. Remove from the oven and let cool in the pan for 10 minutes before transferring to a wire rack to cool completely before frosting.

Prepare one batch of French Buttercream: combine 4 large egg yolks, 1 cup (200g) granulated sugar, ¼ cup (59mL) water, and 1 cup (227g) unsalted butter, then add 2 teaspoons vanilla extract, ¼ to ½ teaspoon almond extract, and ¼ to ½ teaspoon cotton candy extract once the buttercream is ready. Divide the frosting evenly into three bowls. Tint two bowls with gel food coloring to create pink and blue frosting by adding 1–2 drops at a time, adjusting the intensity as needed; add purple to balance if the colors appear too warm. Leave the third bowl uncolored, allowing the natural yellow tint from the egg yolks to show. Lay a piece of plastic wrap on a flat surface and pipe three lines of frosting—one of each color—side by side. Roll the plastic wrap into a log, twist the ends like a candy wrapper, then cut off one end. Insert the wrapped frosting into a pastry bag fitted with a large round piping tip. Pipe swirls of frosting atop each cooled cupcake and finish with a sprinkle of sparkly clear sugar. You may need to repeat the piping process twice to use all the frosting.