Heat your oven to 350°F (175°C). Coat a 10-cup Bundt pan generously with butter or nonstick baking spray, covering every curve and edge. Sprinkle a little flour inside, then shake out any extra to help the cake release easily after baking.

Place the flour, baking powder, baking soda, salt, and ground cinnamon into a bowl. Stir everything together until the ingredients are evenly combined, then keep the bowl nearby.

In a large bowl, beat the softened butter, granulated sugar, and brown sugar until the mixture looks pale and creamy. Mix in the eggs one at a time, blending well after each addition. Add the vanilla extract, sour cream, milk, and vegetable oil, then stir until the mixture is silky and fully combined.

Add the dry ingredients to the wet mixture a little at a time. Stir gently after each addition until no streaks of flour remain. Stop mixing as soon as the batter comes together to keep the cake light and moist.

Spoon or pour the batter into the prepared Bundt pan, spreading it into an even layer. Lightly tap the pan on the countertop a few times to release any large air pockets.

Place the pan in the oven and bake for 45 to 50 minutes. Check for doneness by inserting a toothpick into the center. If it comes out clean or has only a few moist crumbs, the cake is ready. Let it rest in the pan for about 15 minutes, then carefully invert it onto a wire rack to cool.

While the cake is still slightly warm, brush the outside with melted butter. Combine the granulated sugar and cinnamon in a small bowl, then sprinkle the mixture over the cake, gently pressing it where needed so it coats the surface well.

Whisk together the powdered sugar, vanilla extract, dulce de leche, and enough milk or cream to make a smooth glaze. Drizzle it over the cooled cake and allow it to set for several minutes before slicing and serving.