Preheat oven to 350 degrees. Butter an 8 by 8-inch baking dish, line with two sheets of parchment paper while leaving a 2-inch overhang on all sides. Butter parchment paper.
Add butter, unsweetened chocolate and semi-sweet chocolate to a large microwave safe mixing bowl.
Heat in microwave in 20 second intervals stirring well between intervals until melted and smooth. Let cool slightly.
Mix in granulated sugar, brown sugar and salt.
Mix in eggs one at a time then blend in egg yolk and vanilla. Mix in flour.
Pour batter into prepared baking dish and spread into an even layer. Bake in preheated oven 28 - 33 minutes until toothpick inserted into center comes out with crumbs.
Remove from oven and cool completely on a wire rack (I like to cover the pan with plastic wrap about halfway through cooling so they don't dry at all).
For the Frosting: In a medium mixing bowl using an electric hand mixer blend together powdered sugar, cocoa powder, butter half and half and vanilla.
Add in a little more half and half, 1 tsp at a time, to thin if needed. Mix until frosting is slightly fluffy.
Spread over cooled brownies then lift brownies out using parchment overhang.
Cut into squares and store in an airtight container (these are pretty rich which is why I listed so many servings for this small pan size).