Preheat oven to 350°F. Grease 9x13 inch baking pan and set aside.
In a medium bowl, whisk together flour, baking powder, and salt and set aside.
In a large pot on medium low heat, melt butter. Remove from heat and add cocoa. Stir or whisk until smooth, removing most lumps by stirring against the side of the pot.
Add sugar, eggs, and vanilla into the pot and stir in. Using a spatula or wooden spoon, fold in flour mixture and nuts if using, until just combined.
Pour into prepared baking pan and spread evenly.
Bake 30-35 minutes. Cool.
Fudgy Cocoa Frosting
In a medium bowl, combine melted butter and cocoa. Add milk, vanilla, and powdered sugar, and blend until smooth.
Spread frosting on cooled brownies. Slice and serve.
Notes
Store brownies in an airtight container on the counter for up to 3 days or in the refrigerator for 5 days.
To freeze, individually wrap in plastic film and transfer to a zip bag. Store in the freezer for up to 1 month.
Thaw at room temperature, or enjoy warm by microwaving for 30 seconds.