Preheat the oven to 350°F. Line two 9x13 baking pans with parchment paper, allowing the paper to slightly overhang the edges for easy removal. Lightly spray the parchment and sides of the pans with baking spray.
In a medium bowl, sift together the flour, baking powder, baking soda, and salt. Set aside.
3 cups all-purpose flour, sifted, 3 tsp baking powder, ½ tsp baking soda, ½ tsp salt
In a large mixing bowl, with a hand mixer or stand mixer, beat the softened butter, oil, and granulated sugar on medium speed for 2-3 minutes, until smooth and slightly fluffy.
3/4 cup unsalted butter, softened (1 1/2 sticks), 3/4 cup vegetable oil, or canola or olive oil, 1 1/2 cups granulated sugar
Add the eggs, vanilla, and milk and mix until well combined.
4 large eggs, room temperature, 1 tbsp vanilla extract, 1 1/2 cups whole milk
Add the dry ingredients to the wet ingredients and gently mix just until incorporated. (Be sure to NOT overmix the batter.)
Divide the batter evenly between the prepared pans and spread it into an even layer.
Bake at 350℉ for 18-20 minutes, or until the cake has become firm, the top springs back to the touch, and a toothpick or fork inserted in the center of the cake comes out clean.
Allow the cakes to cool in the pans for 5 minutes. Carefully lift the cakes out using the parchment paper and transfer them to wire cooling racks. Let cool completely before assembling.