Place oven shelf in center position and heat oven to 350 degrees F. Generously butter or grease an 8-inch baking pan, dust pan with flour, knocking out excess; set aside. Spread nuts on rimmed baking sheet and toast in oven, shaking pan occasionally, until fragrant and deep golden brown, about 10 to 12 minutes. Transfer nuts to a small bowl; set aside to cool.

In a medium saucepan over low heat, melt 6 tablespoons butter; let cool. Add 1/4 cup powdered sugar, cocoa, and salt and beat until completely incorporated. Add egg and beat until well blended. Stir in graham cracker crumbs, coconut and toasted nuts until well combined. Press mixture evenly into bottom of prepared baking pan. Bake until base begins to darken slightly, about 20 minutes. Remove from oven and let cool completely in pan, set on a wire rack.

Bring water to a boil in bottom of double-boiler. Place 2 tablespoons of butter and bittersweet chocolate in top of double-boiler set over steaming water and melt gently while stirring. When mostly melted, remove from heat and continue stirring until mixture is completely melted and smooth. Set aside and let cool to lukewarm.

In a medium bowl, using an electric mixer on medium speed, beat remaining 1/2 cup (1-stick) butter, remaining 2 cups powdered sugar, milk, and vanilla until smooth and fluffy. Spread mixture evenly over cooled crust. Spread melted chocolate mixture evenly over buttercream filling. Cover and refrigerate for at least 2 hour before cutting into squares. Stored in an airtight container, in the refrigerator, for up to 3 days.

Number of bars will depend on size you cut them.