For the browniesPreheat oven to 350 degrees F. Lightly grease a 9x13 baking dish.
Combine the melted butter, sugars, and vanilla in a large bowl. Beat in the eggs, one at a time, mixing well after each, until thoroughly blended.
In a separate bowl, combine the flour, cocoa powder, and salt. Gradually stir the flour mixture into the egg mixture until blended. Spread the batter evenly into the prepared baking dish.
Bake in the preheated oven until an inserted toothpick comes out clean, about 25 to 35 minutes. Remove, and cool pan on a wire rack before frosting.
For the frostingMelt your chocolate chips if you haven’t already and set them aside to cool. You don’t want it so cool that it hardens, but you can get it close to room temperature and still have it be soft and stir-able.
In a small saucepan, whisk together sugar, flour, cocoa, and milk; bring to a boil, whisking frequently. Boil 1-3 minutes or until thickened like a thinned pudding. Remove pan from heat and strain mixture into a small bowl to get rid of any small flour clumps that may have formed. Cool completely in the refrigerator or freezer.
When the chocolate flour mixture is cooled, beat the butter with an electric mixer until creamy, about 1 minute. Beat in the cooled chocolate mixture until well combined fluffy, about 1 minute. Finally, add in melted chocolate and beat again until well combined and fluffy, about 2 minutes.
Spread frosting on top of the brownies, sprinkle with mini chocolate chips if using, cut, and serve! I opted to make cute little circles, but regular squares are just as delicious!