Melt chocolate and shortening in microwave, checking every 30 seconds to see if the shortening is melted. Do not let chocolate burn. Stir until chocolate is completely melted. Stir in sugar. Cool chocolate mixture until lukewarm. Add eggs and mix well. Add flour and nuts if using, and stir until well blended. Spread into a greased 13 x 9 inch pan. Bake at 350F for 30 to 35 minutes. Do not overbake. Score top of brownies a few minutes after you remove them from the oven. Don't cut all the way through the brownie until they are completely cool.