Preheat oven to 350° degrees Fahrenheit. Liberally grease a 10-cup Bundt pan with cooking spray or butter and flour the pan. Set aside.
Meanwhile, in the bowl of a stand mixer, combine the oil, milk, granulated sugar, and vanilla extract with the paddle attachment until incorporated. Add in the flour, baking powder and soda, and the salt and mix until well blended. Stir in the rainbow jimmies until combined.
Pour the batter (it will be thin and runny) into the prepared Bundt pan. Bake for approximately 35-45 minutes or until a toothpick inserted near the center comes out clean or with moist, but not wet, crumbs. Cool for about 15 minutes, then carefully invert the cake onto a wire rack to cool completely.
After the cake has cooled, make your frosting. In a large bowl, combine the softened butter, cream cheese, and vanilla and mix with a handheld electric mixer until soft and creamy. Add in the confectioners' sugar until well mixed. Spoon the frosting into a disposable piping bag or Ziploc bag (you do not need a piping tip for this). Snip off the corner of the bag and drizzle stripes onto the cake.
Store the cake covered airtight in the fridge.