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Preheat the oven to 325, and grease an 8x8 cake pan. Line the bottom of the pan with parchment paper for easy removal, and set aside.
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In a large bowl, whisk together the brown sugar, melted butter, and canola oil until combined, and then mix in the egg and vanilla extract. In a small bowl, stir together the flour, salt, baking powder, and baking soda. Stir the flour mixture and the buttermilk into the egg mixture in alternating additions, starting and ending with the flour mixture. You want to mix well between each addition until everything is just combined and you have a smooth batter.
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Spread the batter evenly into your prepared cake pan. Dollop the caramel sauce over the top, and gently swirl it into the cake. Optionally, sprinkle flakey salt over the top. Bake for 18-20 minutes, or until a cake tester comes out with just a few crumbs clinging to it. Optionally, you can use an instant read thermometer that should read 200 degrees when the cake is done. Cool the cake completely before adding the topping.