Break or cut chocolate into 1-inch pieces and place in heatproof bowl. Gently melt chocolate in double boiler. Remove from heat; set aside.
Coat an 8-inch square non-stick baking pan with cooking spray; set aside. In large bowl combine preserves, apple butter, honey, eggs, egg whites, salt and vanilla extract. Whisk to thoroughly combine. Whisk in flour until smooth.
Slowly pour melted chocolate into batter, whisking continuously. Spread batter in pan. Sprinkle with walnuts.
Bake 20 minutes or until springy to the touch. Cool on rack. Cut into 24 bars.