Preheat oven to 350°F (176°C). Prepare a 12x18 inch sheet pan with non-stick baking spray.

In a medium bowl, whisk together flour, baking powder, and salt. Set aside.

In a large mixer bowl, beat together butter, vegetable oil, and sugar until light and fluffy, about 3-4 minutes.

Add eggs one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed.

Add half of the dry ingredients to the batter and mix until just combined.

Slowly add the milk and vanilla extract, mixing until well combined.

Add the remaining dry ingredients and mix until smooth. Be careful not to overmix.

Spread the batter evenly into the prepared pan and bake for 12-15 minutes, or until a toothpick inserted into the center comes out clean.

Remove the cake from the oven and let it cool completely in the pan.

To make the frosting, beat together the butter and shortening until smooth.

Add half of the powdered sugar and salt, and mix until smooth.

Add the vanilla extract, corn syrup, and 4 tablespoons of water or milk. Mix until smooth.

Slowly add the remaining powdered sugar. Add more water or milk, a tablespoon at a time, if needed to reach desired consistency.

If desired, divide the frosting and color portions with gel icing colors.

Spread the frosting evenly over the cooled cake and top with sprinkles, if using.