Preheat oven to 350°F (180°C). Grease a 9×13 inch baking pan. In a large bowl, mix together melted butter, 1 cup peanut butter, granulated sugar, brown sugar, salt, vanilla, and eggs until well combined.
In a medium saucepan, melt the ¼ cup butter over low heat. Add the milk and cocoa powder, whisking until smooth. Remove from heat. Gradually stir in the powdered sugar, one cup at a time. Mix in the vanilla.