Preheat oven to 350 degrees F. Line a square baking dish with parchment or foil, or spray with cooking spray.
Combine the butter and flour in a large mixing bowl.
Add the sugar and whisk.
Add the eggs, and vanilla and whisk again.
Stir in the flour and salt. Whisk until just combined.
Pour into your prepared baking pan and bake for 20 minutes or until center is set.
Let cool completely.
To make the frosting, put the butter, sugar, cocoa and salt in a bowl. Whisk until combined. If too thick, add milk a tablespoon at a time until you’ve reached a spreadable consistency.
When brownies are cool, spread the frosting on top and immediately add sprinkles.