Preheat oven to 325 degrees. In a mixing bowl, whisk together granulated sugar and brown sugar, then whisk in flour, cocoa powder and salt. Add in melted butter and using a wooden spoon, stir until evenly moistened.
Mix in eggs then egg yolk and vanilla. Pour mixture into a buttered 8 by 8-inch baking dish and spread into an even layer. Bake in preheated oven 28 - 32 minutes until toothpick comes out free of batter. Allow to cool completely.
For the Dulce de Leche-Salted Peanut Frosting:
In a small saucepan set over medium-high heat, bring butter, milk and light brown sugar to a boil, stirring frequently.
Allow mixture to boil for 1 minute then remove from heat, stir in dulce de leche and vanilla. Add in powdered sugar and whisk until well blended.
Stir in peanuts and pour mixture over cooled brownies and spread into an even layer. Transfer to refrigerator or allow frosting to set at room temperature then cut into squares and store in an airtight container.