In a microwave, melt butter and chocolate; stir until smooth. Cool slightly. Meanwhile, in a large bowl, beat the eggs, sugar and vanilla. Add the chocolate mixture and mix well. Combine flour and salt; gradually add to chocolate mixture. Stir in nuts.
Spread into a greased 13x9-in. baking pan. Bake at 350° for 16-20 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack.
In a small bowl, cream butter, sugar and extract until light and fluffy. Add enough cream until frosting reaches spreading consistency. Frost cooled bars. Melt chocolate and butter; drizzle over frosting. Cut into bars.