1. Prepare a 12×18 inch sheet pan/jelly roll pan with non-stick baking spray. Preheat oven to 350°F (176°C).
2. Combine the flour, baking powder and salt in a medium sized bowl and set aside.
3. Add the butter, sugar, and oil to and large mixer bowl and beat together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time.
4. Add the eggs two at a time, mixing until mostly combined after each addition. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
5. Add half of the dry ingredients to the batter and mix until mostly combined.
6. Slowly add the milk and vanilla extract and mix until well combined.
7. Add the remaining dry ingredients and mix until well combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the batter.
8. Spread the batter evenly into the prepared pan and bake for 12-15 minutes, or until a toothpick comes out clean.
9. Remove the cake from the oven and allow to cool.
10. To make the frosting, beat the butter and shortening together until smooth.
11. Slowly add half of the powdered sugar and the salt and mix until smooth.
12. Add the vanilla extract, corn syrup and 4 tablespoons of water or milk and mix until smooth.
13. Slowly add the remaining powdered sugar and mix until smooth. Add additional water or milk, as needed to get the right consistency of frosting.
14. Color the frosting to your desired shade. I used a little bit of pink, peach and ivory gel icing colors.
15. Spread the frosting evenly onto the cooled cake and then top with sprinkles. Cover the cake well and store at room temperature. Cake is best when eaten within 3-4 days.