Preheat the oven 350°F. Lightly spray (2) 9-inch cake pans with baking spray. 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 the flour, baking powder, baking soda, and salt. Set aside.
In the body of a stand mixer with the paddle attachment mix the butter with the oil until combined.
Add the granulated sugar and brown sugar. Beat them together on medium speed for about 3 minutes.
Then, add the eggs one at a time until fully mixed in. Stir in the vanilla extract.
Add half of the dry ingredients and stir it in until combined.
Next, add half of the buttermilk, stir until combined.
Repeat with the remaining dry ingredients and 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 for 22-28 minutes, until a toothpick inserted into the center comes out clean. Let cool completely in the pan.
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 light brown sugar, dark brown sugar, butter, and 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 the evaporated milk then the heavy cream, whisk to combine.
Bring to a simmer and simmer for 4 minutes, it will start to thicken. Immediately pour into a heatproof mixing bowl.
Using an electric hand mixer, add the powdered sugar a little at a time until fully mixed in. Then mix in the vanilla extract.
When the frosting is done, we 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.