In a nonstick skillet, melt butter over medium-high. Add nuts, sugar and a pinch of salt.; stir until sugar browns, 5 minutes. Transfer to plate. Coat 5-by-9-inch loaf pan with cooking spray; line with plastic wrap, leaving 3-inch overhang. In food processor, puree avocado flesh, condensed milk, lemon juice and pinch salt. Whip cream to stiff peaks; fold in avocado puree and nuts. Pour into pan. Cover with plastic overhang; freeze until firm, 4 hours. Unmold onto platter. Slice.