Line a 13x9x2-inch pan with foil, extending foil over pan edges. In a medium saucepan, heat 1/2 cup butter over low heat. Stir in 1/4 cup sugar, 1 tablespoon flour, and one egg. Cook and stir over medium-low heat for 3 to 4 minutes or until blended and slightly thickened. Stir in graham cracker crumbs and chopped pecans. Press mixture into prepared pan; chill.
In a medium saucepan, melt chocolate over low heat, stirring constantly. Set aside. In another large saucepan, stir together the cornstarch and the 2/3 cup sugar until cornstarch lumps are gone. Whisk in whipped cream and butter; bring to a simmer over medium heat, stirring constantly; cook and stir for 1 to 2 minutes or until mixture is thickened and bubbly. Slowly stir about 1/4 cup hot whipped cream mixture into beaten eggs. Slowly whisk beaten eggs into simmering cream mixture. Cook and stir over low to medium-low heat until mixture is thickened (don't boil). Whisk in melted chocolate. Pour chocolate mixture over crust in prepared pan.
Chill for 2 hours. Remove bars from pan and cut into squares or triangles.