Prepare three 8-inch cake pans by lining the bottoms with parchment paper circles and greasing the sides. Preheat the oven to 350°F (176°C). In a medium bowl, whisk together the flour, baking powder, ground cinnamon, and salt, then set aside. In a large mixing bowl, combine the butter, sugar, vegetable oil, and vanilla extract, beating on medium-high speed until the mixture is light in color and fluffy, about 3 to 4 minutes—don’t rush the creaming process. Add the eggs one at a time, mixing after each addition until mostly combined, scraping down the sides of the bowl as necessary to ensure everything is well incorporated. Next, add half of the dry ingredient mixture to the wet ingredients and mix until mostly combined. Gradually pour in the milk and mix until fully incorporated; the batter may appear curdled, but this is normal. Add the remaining dry ingredients and mix until smooth and well combined, scraping the bowl as needed, being careful not to overmix. Evenly divide the batter among the prepared pans and bake for 22 to 25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Remove the cakes from the oven and let them cool in the pans for 2 to 3 minutes before transferring to wire racks to cool completely. To prepare the frosting, beat the butter in a large bowl until smooth and creamy. Slowly add half of the powdered sugar, mixing well to combine. Add the vanilla extract and ground cinnamon, mixing thoroughly. Pour in 4 tablespoons of water or cream along with the salt, beating until smooth. Gradually add the remaining powdered sugar, mixing until the frosting is smooth and reaches the desired consistency, adding more water or cream if necessary. To assemble the cake, level the tops of the cakes with a serrated knife to remove any domes, ensuring each layer is flat. Place the first cake layer on a serving plate or cake board, then spread about 1 cup of frosting evenly over it. Add the second cake layer and top with another cup of frosting. Position the final cake layer on top and frost the outside of the cake evenly. If you need guidance, refer to a tutorial on how to achieve a smooth frosting finish. Pipe swirls of frosting around the top edge of the cake using a piping tip like Ateco 849. Place a churro atop each swirl and arrange additional churros around the base of the cake. Optionally, sprinkle the entire cake with cinnamon sugar for an extra touch. Store the finished cake in an airtight container; it’s best enjoyed within 3 to 4 days.