Preheat the oven to 350°F. Line an 8x8 baking pan with parchment paper.
Mix together the chocolate and butter. Heat, using your preferred method (I recommend using the microwave in 30-second increments) stirring frequently until they are both melted and combined.
Stir in the sugars and whisk well.
Add in the eggs and vanilla, then whisk vigorously for about 2 minutes.
Add in the flour, cornstarch salt, and cocoa powder together into the wet ingredients. Stir until fully combined and pour into the baking pan.
Bake for 30 - 35 minutes, or until a toothpick comes out nearly clean, with just a few crumbs sticking to it.
Frosting
While the brownies are cooling, beat the butter and sugar together until well combined.
Stir in the vanilla, salt, and flour. Mix until just combined.
Add in some milk, just until the mixture reaches your desired spreadable consistency.
Stir in chocolate chips.
Assembly
Once the brownies are fully cooled, spread the frosting over the top of the brownies and cut into squares.
Notes
Heat treat the flour for the frosting before using. Heat the oven to 300F and place the flour on a lined baking sheet. Bake for 10 minutes and allow to cool completely