Combine sweetened condensed milk and corn syrup in a large (2 to 2½ quart) microwave safe bowl.
Microwave on 50% power, stopping and stirring every 2 minutes initially then more frequently as needed during the middle of the cooking time for a total of 10 to 12 minutes or until the mixture is thick and caramel colored. It is important to watch the mixture as it cooks and stir frequently to avoid boil over. The mixture develops a somewhat curdled appearance that will smooth out when stirred. Whisk in butter and set aside.
Preheat oven to 325° F.
Butter a 9×13-inch baking pan.
Cream butter and sugar until fluffy. Beat in eggs and vanilla. Combine flour, cocoa, and salt. Add dry ingredients into creamed mixture; mix until well blended.
Spread ½ of mixture in the prepared pan. Drop spoonfuls of Dulce de Leche evenly over this layer. Cover with remaining brownie batter. Sprinkle nuts evenly over the top.
Bake 35 to 40 minutes. Cool and cut into squares.