Add the dry ingredients: 3 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1 teaspoon fine sea salt to a bowl of a standing mixer with a paddle attachment.
In a separate bowl combine the wet ingredients; 1 cup melted butter, 1/4 cup vegetable oil, 1 cup granulated sugar, 1 cup brown sugar, 5 eggs, 1 tablespoon of vanilla and 1 cup of buttermilk.
In a small bowl combine 1 cup dark brown sugar and 1 tbsp of ground cinnamon.
Turn the standing mixer on low and slowly pour the wet ingredients into the dry ingredients. Mix until combined.
Spray a bundt cake pan with non-stick spray and sprinkle some flour in to make sure the cake does not stick.
Spoon half of the batter into the bundt cake pan.
Sprinkle the brown sugar and cinnamon mixture evenly on top of the batter inside the bundt cake pan.
Finish spooning the rest of the batter into the bundt cake pan.
Place the cake in the preheated oven and bake for 55 minutes.
When the cake is done, carefully flip the pan upside down onto a parchment paper so that the cake cools and releases from the bundt cake pan.
Once the cake is cooled, serve. We love to serve it with homemade caramel sauce.