In a large saucepan, melt the butter over medium heat, stirring frequently until it turns a golden brown color and has a nutty aroma, about 5-7 minutes. Be careful not to burn it.
Remove the saucepan from heat and stir in the dulce de leche, vanilla extract, and salt until well combined.
Add the Rice Krispies cereal to the mixture and stir until the cereal is evenly coated. If using, fold in the mini marshmallows and chopped nuts.
Lightly grease a 9x13-inch baking dish with butter or cooking spray. Pour the mixture into the dish and press it down firmly with a spatula or your hands to create an even layer.
Allow the treats to cool at room temperature for about 30 minutes before cutting into squares.