Preheat the oven to 350 degrees. Line the bottom of a 9x13 baking dish with parchment paper. Spray sides and paper with cooking spray and set aside.
Add the butter to a medium saucepan over low-medium heat. Once the butter is melted, whisk in the cocoa powder. Remove from heat and set aside.
In a stand mixer with the whisk attachment (or using a hand mixer) whisk together the eggs and sugar for 2-4 minutes, until they become pale in color and double in volume.
While the mixer is whisking on low, drop the chocolate and butter mixture into the mixer in spoonfuls until the mixture is combined. Add vanilla extract. Add the flour and salt and mix on medium speed just until combined.
Remove the mixer from the stand and stir in the walnuts (optional) then spread the mixture into the greased pan. Bake for 30-35 minutes, or until the top looks set and an inserted toothpick comes out clean.
Let the brownies cool for 15 minutes in the pan, then transfer them onto a wire rack to cool for 2 hours.
Once the brownies are room temperature, make the frosting by combining all ingredients in a large bowl. Start with 2 tablespoons of milk and use a hand mixer on low to combine everything. You will likely need to add milk, only add milk 1 tablespoon at a time. Whip the frosting to a thick consistency and spread evenly on the cooled brownies. Serve!
