Bring the butter to a boil in a 3 quart saucepan over high heat. Turn off heat and immediately add sugar to the butter in your saucepan. Stir with whisk until thoroughly incorporated or sugar is dissolved. Use a spatula to scrape the sides when necessary. Add eggs one at a time stirring well after each egg. Add salt. Add flour 1/3 cup at a time mixing well after each addition. Add Cocoa Powder 1/3 cup at a time mixing well after each addition. Add baking powder. Pour/spread into prepared 12 x 17 x 1 baking dish. Bake for EXACTLY 20 minutes. Cool on wire rack for 10 minutes before slicing. I use a credit card for slicing my brownies. Safe on thin aluminum and safe for kids. Just don't give them a current one.
Mix the milk, powdered sugar, butter, cocoa & salt together with a fork till combined. Beat with an electric mixer on high for 1 minute till light and fluffy.
Dust with powdered sugar or sprinkle with nuts if desired.