Preheat oven to 350°F and line a 9" x 13" baking pan with parchment paper. In a small pot, melt the chocolate and butter together over low heat, stirring constantly. Set aside and allow to cool for a moment. Add the sugar and mix well. Set aside. In a food processor or coffee grinder, pulse nuts until they are fairly fine, but not perfectly ground. Add nuts to chocolate mixture and mix well. Add eggs a little at a time, stirring very quickly so as to mix the eggs in before the heat of the chocolate cooks them. Once the eggs are blended in, stir in salt. Pour the batter in the pan and sprinkle apples over top. Bake for 35 minutes. Cool completely before removing from pan and cutting into squares.