Preheat oven to 350 degrees F. Grease the inside of a 9-inch x-13inch baking dish and set aside.
Melt chocolate & butter together in a microwave-safe bowl for 30 seconds at a time, stirring between each heating. (Or you can use a double boiler on the stovetop.)
Transfer chocolate mixture to a large mixing bowl. Beat in sugar and brown sugar,
Add eggs and vanilla to the bowl and mix until combined. Scrape the side of the bowl as needed.
Mix flour, cocoa powder, and salt in another mixing bowl. Add dry ingredients to wet ingredients and mix until just combined.
Pour batter into your prepared baking dish and bake for 35-40 mins.
Remove brownies from the oven. Cool in the dish on a wire rack until room temperature.
Mix together icing ingredients until smooth and fluffy. Spread over brownies in an even layer. Let rest until icing sets, and then cut into 12 brownies.
Notes
You can use chocolate chips instead of a chocolate bar, just be sure to measure by weight.