In a large bowl, cream butter and sugar until light and fluffy. Beat in eggs and vanilla. Combine the flour, cocoa, salt, and espresso powder; gradually add to the creamed mixture.
Spread into a greased 13-in. x 9-in. baking pan lined with parchment paper that is hanging off of two sides. Bake at 350° for 25-30 minutes or until the top is dry and the center is set. Cool completely.
While your brownie is baking, make your frosting. In a stand mixer, beat your butter and sugar, adding milk to consistency. The frosting should keep it's shape, but not be too firm.
When your brownie is cooled completely, spread your buttercream evenly over it. Melt 6 ounces of dark chocolate in the microwave in 30 second intervals. Spread the melted chocolate evenly over the frosting. Chill in your refrigerator for at least an hour.