Grease a 13x9x2-inch baking pan. Set aside. In a large bowl, beat butter on medium to high speed for 30 seconds. Add the 2 cups sugar and beat until well combined. Add the 3 eggs, one at a time, beating well after each addition.
Stir in the melted chocolate and nuts, if you like. Stir in flour and salt. Measure 1 cup of the chocolate mixture and set aside. Spread remaining mixture into prepared baking pan. Bake in a 325 degree F oven for 20 minutes.
In a medium bowl, beat cream cheese, the 3/4 cup sugar, and the vanilla with an electric mixer on medium to high speed until combined. Add the 2 eggs all at once and beat on low speed until just combined.
Carefully spread the cream cheese mixture over the hot partially-baked chocolate layer in the pan. Stir milk into the reserved chocolate mixture. Dot the reserved chocolate mixture over cream cheese layer. Using a knife, carefully swirl chocolate into cream cheese for a marble effect.
Bake in a 325 degree F oven for 25 to 30 minutes more or until cream cheese layer appears set. Cool completely in pan on a wire rack. To serve, cut into squares. Store any leftovers in the refrigerator. Makes 32 bars.