Preheat oven to 350° F. Spray two 8- or 9-inch-round cake pans with nonstick cooking spray. Dust with baking cocoa, shaking out excess.
Combine flour, granulated sugar, baking cocoa, baking powder, baking soda and 1/4 teaspoon salt in large mixer bowl. Add milk, water, oil, eggs and 2 teaspoons vanilla extract; beat until moistened. Beat for 2 minutes. Pour into prepared pans.
Bake for 28 to 30 minutes or until wooden pick inserted in center comes out clean. Cool in pans on wire racks for 10 minutes; remove to wire racks to cool completely.
Beat butter, dulce de leche, remaining teaspoon vanilla extract and 1/4 teaspoon salt in large mixer bowl until creamy. Stop the mixer and add half of the powdered sugar; beat on low until well blended. Beat in remaining powdered sugar until blended. Continue to beat on medium speed for about 3 minutes or until fluffy and lighter in color.
Place dollop of buttercream in the center of a cake pedestal or serving platter so the cake won’t slip. Place one cake layer over dollop of buttercream. Spread about 1 cup buttercream over top, spreading to edge of cake. Top with second cake layer, spreading remaining buttercream over top of cake. If desired, warm up some of the remaining dulce de leche in a small bowl and swirl into buttercream on top of cake.