Apapted from Cook's Illustrated and Southern Living via Crunchy, Creamy, Sweet
2 cups plus 2 tablespoons flour
1/2 teaspoon baking soda
1/2 teaspoon salt
12 tablespoons butter (1 1/2 sticks), melted and cooled
1 cup packed dark brown sugar (I added 1 tablespoon molasses to 1 cup light brown sugar)
1/2 cup granulated sugar
1 egg yolk
2 teaspoons vanilla
1/2 cup butter (1 stick)
1/3 cup milk
6 tablespoons unsweetened cocoa
2 cups powdered sugar
1 teaspoon vanilla
Preheat your oven to 325º.
Line a 13 x 9 baking dish with non-stick foil (or use regular foil and grease the foil).
In a medium mixing bowl, whisk together the dry ingredients ( flour, soda and salt ). Set aside.
In a large mixing bowl, combine both sugars and melted butter and whisk until smooth. Add the egg, egg yolk and vanilla and whisk again. Gently fold in the dry ingredients with a wooden spoon. Do not over-mix.
Smooth the batter into the prepared pan and bake for 27 - 30 minutes, until the top is golden brown.
Let cool for few minutes in the pan and the lift out and place on a rack to cool completely.
Make frosting by combining butter, milk and cocoa in a saucepan. Whisk and cook for 4 - 5 minutes, until the butter has melted. Remove the saucepan from heat. Pour the cocoa mixture into a large mixing bowl and beat with an electric mixer for 1 minute. Add the powdered sugar, 1/2 cup at a time. Add vanilla and beat until smooth. You may need to chill slightly to get it to icing consistency.
Frost cooled bars.