Preheat the oven to 350°F. Lightly spray a 9x13x3-inch baking dish with baking spray and set aside.
In a medium-sized bowl, stir together 3 cups all-purpose flour, 2 teaspoons baking powder and 1/2 teaspoon baking soda. Set asideNOTE: if using unsalted butter, you'll also add one teaspoon of salt here.
In the body of a stand mixer with the paddle attachment (or a bowl with an electric hand mixer), cream together the 1/2 cup (1 stick) salted butter, softened to room temperature, 1/2 cup vegetable oil and 1 3/4 cups granulated sugar for 3 minutes until fluffy.
Add in 3 large eggs, room temperature (one at a time) until fully mixed in. Add 2 large egg yolks, room temperature and mix them in along with 1 1/2 teaspoons vanilla extract.
Add half of the dry ingredients you stirred together earlier and stir it in until combined.
Next, add half of the buttermilk, and stir until combined. Repeat with the remaining dry ingredients and buttermilk, scraping down the sides as needed.
Pour the batter into the prepared baking dish and smooth out the top.
Bake for 35-40 minutes, on the middle oven rack, until a toothpick inserted into the center comes out clean. If you see the cake is browning too much, cover it loosely with foil.NOTE: Baking times can vary depending on your oven, altitude, and many other factors. Please go by doneness not just by time.
Let cool completely.
For the frosting:
Once the cake is cool, make the frosting. In the body of a stand mixer with the paddle attachment (or in a large bowl with an electric hand mixer) add 1 cup (2 sticks) salted butter, softened to room temperature until smooth.
Add 1/2 cup unsweetened cocoa powder and slowly mix it in until combined.
Add 4 cups powdered sugar, a little at a time, mixing the first batch in completely before adding more. The mixture will be thick.
Add in 1 teaspoon vanilla extract and 4 Tablespoons of heavy cream. Mix until smooth. If it appears too thick, add an additional 1 Tablespoon of heavy cream. Once mixed in, turn the speed up to medium-high and whip for 3 minutes until light and fluffy. Scrape down the sides as needed.
Frost the cake with an even layer of frosting. Add sprinkles if using. Cut and serve.
Notes
Please refer to my FAQ’s and ingredient list above for other substitutions or for the answers to the most common questions.
When picking your baking dish, choose one that is deeper, like a 9x13x3-inch baking dish rather than a normal 9x13x2-inch baking dish. This way there is enough room for the cake to rise and be able to add frosting on top.
Unsalted butter can be used. Just add one teaspoon of salt to the dry ingredients.
You gotta use buttermilk in this to keep it moist. I do not recommend milk. You could maybe substitute with heavy cream.
Leftovers can be stored at room temp in an airtight container or wrapped appropriately for up to 5 days. You can keep it longer by freezing it for up to 3 months.