Microwave butter and bittersweet chocolate in a 2-quart glass bowl at HIGH 1 1/2 minutes or until chocolate is melted, stirring mixture twice. Whisk in 2 cups sugar, eggs, and vanilla extract.
Toss together 1 tablespoon flour, walnuts, and chocolate morsels.
Stir remaining flour and salt into sugar mixture. Add walnut mixture. Spread batter into a lightly greased 13- x 9-inch pan.
Bake at 350° for 30 to 40 minutes. (A wooden pick inserted in center will not come out clean.) Cool on a wire rack.
*Bourbon may be substituted for vanilla extract.