Preheat your oven to 325°F and place the rack in the middle position. Prepare a 12-cup Bundt pan by generously spraying the inside with nonstick baking spray or brush generously with my Homemade Cake Release. In the bowl of your stand mixer fitted with a whisk attachment, add butter and sugar and mix on high for 3-4 minutes, until pale and fluffy.
Next, add eggs one at a time, combining well after each addition, and scraping down the sides and bottom of the bowl as needed.
Reduce the mixer speed to low and add flour, salt, and cinnamon in two additions. Be careful not to over mix.
Add 4 tablespoons dulce de leche to a small microwave-safe bowl and heat it in the microwave on 50% power for about 1 minute, until softened.
Add the softened dulce de leche, sour cream and vanilla to the batter and mix on low until just combined. Make sure to scrape the bottom and sides of the bowl so everything is evenly mixed.
Pour the batter into the prepared pan and bake for 1 hour 20 minutes or until an inserted toothpick comes out mostly clean with a few moist crumbs.
Remove the cake from the oven and cool in the pan for 10 minutes. Then invert the cake onto a serving plate and allow it to cool completely, about 1 hour.