Preheat the oven to 350ºF/177ºC and line a cupcake pan with 12-15 liners.

Add the sifted cake flour, baking powder, baking soda, and salt into a bowl and whisk to combine. Set aside.

In the bowl of your stand mixer (a handheld mixer works fine too!), cream the butter for on high for two minutes until it's light and fluffy. Add vegetable oil and sugar, then continue to mix on med-high for another two minutes. Add the whole egg and egg yolk and mix until well combined. Add vanilla and sour cream and mix for one minute on high, scraping down the bowl and paddle once more.

With the mixer on low speed, add in the dry ingredients and mix until just combined, then continue mixing while you add the milk in a slow stream. Mix until just incorporated, then scrape down the sides and bottom of the bowl and give it a few stirs to make sure there are no lumps (without over-mixing). The batter will be thick.

Spoon the yellow cake batter into the cupcake liners, filling each one between ½ and ⅔ full. Bake for 15-18 minutes, or until a toothpick inserted comes out clean. Allow the cupcakes to cool fully before frosting.

In a stand mixer fitted with a paddle attachment, whip the butter on high speed until creamy and light in color, about five minutes. Scrape down the bowl and paddle and turn the mixer to low, then add the powdered sugar, cocoa powder, milk, vanilla extract, and salt. Continue mixing on low speed until fully combined and uniform, 2-3 minutes, scraping down the bowl and paddle as needed.

When the cupcakes are fully cooled, fit a piping bag with Wilton Tip 1M. Fill the piping bag with the chocolate buttercream and pipe a swirl onto each cupcake. Garnish with rainbow sprinkles.