“The cake was just right—not too hard and not too crumbly, and the frosting was just right too—not too sweet.” —Madison, recipe tester, age 8
Have you heard about our Young Chefs’ Club? Members get a themed (and kid-tested) box delivered each month!
Adjust oven rack to middle position and heat oven to 350 degrees. Spray inside bottom and sides of 13-by-9-inch metal baking pan with vegetable oil spray. Line bottom of baking pan with 13-by-9-inch piece of parchment paper.
In medium bowl, whisk together flour, baking powder, baking soda, and salt.
Add eggs to bowl of stand mixer (or large bowl if using handheld mixer). Lock bowl into place and attach whisk attachment to stand mixer, if using. Start mixer on medium speed. Whip eggs until foamy, 1 to 2 minutes. Stop mixer.
Add sugar to eggs and start mixer on medium-high speed. Whip mixture until pale yellow and fluffy, about 3 minutes.
Reduce speed to medium. With mixer running, very slowly pour in melted butter and oil and mix until fully combined, about 2 minutes. Stop mixer.
Carefully add half of flour mixture. Start mixer on low speed and mix until just combined, about 30 seconds. With mixer running, add buttermilk and vanilla and mix until combined, about 30 seconds. Stop mixer.
Add remaining flour mixture. Start mixer on low speed and mix until just combined and no dry flour is visible, about 30 seconds. Stop mixer. Remove bowl from stand mixer, if using.
Use rubber spatula to scrape down sides of bowl and stir in any remaining dry flour. Use rubber spatula to scrape batter into parchment-lined baking pan and smooth top (make sure to spread batter into corners to create even layer).
Place baking pan in oven. Bake until cake is golden brown and toothpick inserted in center comes out with few crumbs attached, 27 to 30 minutes.
Use oven mitts to remove baking pan from oven (ask an adult for help). Place baking pan on cooling rack and let cake cool completely in pan, about 2 hours. (This is a good time to make the frosting!)
Remove cake from baking pan and discard parchment (click here to learn how to remove cake from baking pan). Use icing spatula to spread frosting evenly over top and sides of cooled cake (click here to learn how to frost a sheet cake). Cut cake into pieces and serve.