Preheat your oven to 350°F (175°C). Prepare two 9-inch cake pans with baking spray and a layer of parchment paper on the bottom.

In a bowl, whisk together your flour, baking powder, baking soda, and salt.

In a separate bowl, beat your softened butter and vegetable oil together until smooth.

Add your granulated sugar and brown sugar into the butter mixture and beat until light and fluffy.

Add your eggs one at a time, beating well after each addition, then mix in the vanilla extract.

Add half of your dry ingredient mixture into the batter and mix until just combined, then pour in half of your buttermilk and mix gently. Repeat with the remaining dry ingredients and buttermilk.

Divide your batter evenly between the two prepared pans and bake for 22 to 28 minutes, checking with a toothpick inserted into the center.

In a saucepan, combine your butter, both brown sugars, salt, evaporated milk, and heavy cream. Cook over medium heat, stirring frequently, until the mixture is smooth and fully melted.

Remove the saucepan from the heat and beat in your sifted powdered sugar and vanilla extract until the frosting is creamy and spreadable.

Once your cake layers have cooled completely, spread a generous layer of frosting between them.

Cover the top and sides with the remaining frosting.