Preheat the oven to 350°F. Grease and lightly flour an 8x8 inch pan with gf flour. Cream butter, sugar and egg. Add 1 ounce of melted chocolate, 1/2 cup gf flour and nuts. Place mixture into prepared pan and bake for 20 minutes or until an inserted toothpick comes out clean. Remove from oven and let cool.
Meanwhile combine 2 Tablespoons unsalted butter, powdered sugar, cream and vanilla extract. When cake layer is cool, spread buttercream mixture on top.
Melt remaining 1 ounce chocolate and 1 tablespoon butter; pour over filling. Chill in the refrigerator. Cut into 24 small bars.