cheese and the 1-3/4 cups sugar. beat with an electric mixer on medium speed until smooth. add eggs, one at a time, beating on low speed after each addition just until combined. beat in pumpkin, pumpkin pie spice, vanilla, and salt on low speed just until combined. remove 1-1/4 cups of the mixture.
in a small heavy saucepan, combine the 6 ounces chocolate and the 2 tablespoons butter. cook and stir over low heat until melted. whisk chocolate mixture into the 1-1/4 cups pumpkin mixture. pour over crust, spreading evenly. bake for 15 minutes.
carefully pour the remaining pumpkin mixture over baked chocolate layer, spreading evenly. bake for 40 to 45 minutes 