Preheat oven to 350. Generously grease two 9" round cake pans and dust with flour; set aside.
Combine cake mix, milke, melted butter, eggs, and vanilla in large bowl. Blend on low for 1 minute. Stop blending and scrape sides of bowl; blend for additional 2 minutes on medium. Divide the batter between the two prepared pans. Cook on center rack side by side.
Bake until golden brown and spring back when lightly pressed, about 27-29 minutes. Remove from oven and place on wire racks to cool for 10 miutes. Run knife around edge of each layer and invert onto rack. Invert again onto another rack so that the cakes are right side up. Allow them to cool completely, about 30 mimutes more.
While cakes are cooling, prepare frosting. Place butter and brown sugars into medium sized saucepan over medium heat. Stir and cook until mixture comes to a boil, about 2 minutes. Add the milk, and bring mixture back to a boil. Remove the pan from the heat. Add powdered sugar and vanilla. Beat with a wooden spoon until the frosting is smooth. **You will want to time the cooking of the frosting with the cooling of the cakes. The cakes should be cool, but the frosting should be warm. The frost may start to harden a bit as a it cools. If this happens, place it back on low heat stirring constantly until it softens again.**
Place one cake layer onto a serving platter. Spread the top with the warm caramel frosting. Place the second layer cake on top of the first. Frost the top and the sides of the cake, making cean, smooth strokes. Again, be sure to work quickly, as frosting will set as it cools.
Once frosting has set, slice and serve!
Store covered in plastic wrap or foil at room temperature for up to 1 week.