Preheat your oven to 325°F. Grease and line the bottom of a 9x5-inch loaf pan with parchment

In a large bowl, whisk together the flour, cornstarch, sugar, baking powder, baking soda, and salt

Add the softened butter and beat with a hand mixer on low until the mixture resembles damp sand and no large butter pieces remain, about 1-2 minutes

Drizzle in the oil and mix for another 20–30 seconds until evenly combined

In a separate bowl, whisk together the egg, egg yolk, milk, Greek yogurt, and vanilla extract

Add half of the wet ingredients to the flour mixture and mix on medium speed just until there’s no more loose liquid

Add the remaining wet ingredients and mix only until no streaks remain. Avoid overmixing

Fold in the rainbow jimmies with a spatula, then let the batter rest for 10 minutes so it thickens slightly and is able to hold more sprinkles in place once baked without them dropping or floating to the edges

Give the batter one more gentle fold, then transfer it to the prepared loaf pan and smooth the top

Bake for 35-40 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs

Cool in the pan for 15 minutes before transferring to a wire rack to cool completely

Once your cake is cooled, make your vanilla frosting. Add your room-temperature butter to a mixing bowl and beat with a mixer until smooth

Sift in your powdered sugar and mix until combined. Add in your vanilla extract and milk. If you want your frosting looser, add half a tablespoon of milk at a time, mixing in between, until you get your preferred consistency

To assemble your cake, use a serrated knife to cut the cooled loaf in half vertically so you have two rectangles. Then cut each rectangle in half horizontally, so you now have four thin squares. Place one on your cake plate and pipe or spread a layer of frosting before topping with another cake layer. Repeat until all layers have been stacked, then frost and decorate as desired!