Preheat oven to 325°. In a microwave, melt butter and chocolate; stir until smooth. Cool slightly. In a large bowl, beat eggs and sugar. Stir in Nutella and liqueur. Combine flour and salt; gradually add to chocolate mixture. Fold in 1 cup nuts.
Transfer to a greased 9-in. square baking pan. Sprinkle with remaining nuts. Bake until a toothpick inserted in the centre comes out with moist crumbs (do not overbake), 30-40 minutes. Cool on a wire rack. Cut into bars.