Preheat the oven 350°F. Lightly spray (2) 9-inch cake pans with baking spray (I like the kind with flour in it). Place parchment paper in the bottom of the pans. Spray the top of the paper with baking spray. Set aside.
In a medium-sized bowl, stir together 2 ⅔ cup all-purpose flour, 2 teaspoon baking powder, ½ teaspoon baking soda and ½ teaspoon salt. Set aside.
In the body of a stand mixer with the paddle attachment (or in a bowl with an electric hand mixer) mix ½ cup unsalted butter, softened to room temperature and ½ cup vegetable oil until combined.
Add in 1 cup granulated sugar and ¾ cup light brown sugar, packed. Beat them together on medium speed for about 3 minutes.
Then, add 4 large eggs, room temperature, one at a time, until fully mixed in. Stir in 1 tablespoon vanilla extract.
Add half of the dry ingredients and stir it in until combined.
Next, add 3/4 cup buttermilk, stir until combined.
Repeat with the remaining dry ingredients and 3/4 cup buttermilk, scraping down the sides as needed.
Divide the batter between the two cake pans. Take a spatula and smooth out the top, this will ensure the cakes bake evenly.
Bake on the middle rack for about 22-28 minutes, until a toothpick inserted into the center comes out clean. Let cool completely in the pan.NOTE: Oven times can vary. Please go by doneness, not just by time.
When the cakes are cool and you are ready to assemble, cut any doming or uneven parts off the top of the cake if there are any, and then make the frosting.
Place the 1 cup light brown sugar, packed, 1 cup dark brown sugar, packed and 1 cup unsalted butter (cut into Tablespoons) and ½ teaspoon salt into a medium-sized saucepan.
Place the pot over medium heat and whisk constantly until the butter and the sugars are melted.
Pour in ⅓ cup evaporated milk and ⅓ cup heavy cream, whisk to combine.
Bring to a simmer and simmer for about 4 minutes, it will start to thicken. Immediately pour into a heatproof mixing bowl.
Using an electric hand mixer, add 2 cups powdered sugar, sifted, a little at a time, until fully mixed in. Then mix in 1 teaspoon vanilla extract.
When the frosting is done, you need to work fast to frost the cake while it is still spreadable.
Place one of the cakes top side up onto your serving platter. Add ¾ cup of the frosting and using an offset spatula or something similar, spread it out over the top of the cake.
Place the other cake top on top (you want the flattest side facing up).
Frost the sides and top of the cake as fast as you can before the frosting hardens.
Let the cake sit until the frosting completely hardens (about 2 hours) before slicing to serve.
Video
Notes
You can use a yellow box cake mix if you do not feel like making homemade.
If you cannot find or do not have buttermilk, you can make your own. See above on how to do that.
This can be frozen, see above on how to freeze.
Other toppings can be added, see some of my suggestions at the top.