for the cake:

1/4 cup (1/2 stick) unsalted butter, at room temperature (and boy, do i ever mean that)
1/3 cup vegetable shortening
1 1/4 cups granulated sugar
3 tablespoons packed light brown sugar
3 eggs
1/2 cup buttermilk
1/3 cup grapeseed oil
2 teaspoons clear vanilla extract (and i did make myself use clear vanilla this time, because i wanted a snowy-white cake)
2 cups cake flour
1 1/2 teaspoons baking powder
3/4 teaspoon kosher salt
1/3 cup rainbow sprinkles + around 3-4 tablespoons for sprinkling over top your batter

for the cake soak:

1/4 cup milk
1 teaspoon clear vanilla extract

for the frosting:

1/2 cup (1 stick) unsalted butter, at room temperature (I’m serious about this one, too)
1/4 cup vegetable shortening (don’t freak out; it’ll make you a believer in shortening)
2 ounces cream cheese
1 tablespoon glucose OR 2 teaspoons light corn syrup (I have found both work fine here)
1 tablespoon corn syrup
1 tablespoon clear vanilla extract (important, especially with the frosting, for the stark white color)
1 1/4 cups confectioners’ sugar
1/2 teaspoon kosher salt
pinch baking powder