Place the butter in the top of a double boiler set over barely simmering water.
When the butter is partially melted, add the chocolate.
Heat for 5-7 minutes, or until melted; then whisk until smooth.
Remove the top of the double boiler and set aside to partially cool.
Add the cream cheese to a large bowl; cream using an electric mixer on medium speed, gradually adding in 1 1/4 cups sugar.
Beat in the eggs, one at at time; then blend in the pumpkin, vanilla, spices, and salt until well blended.
Pour slightly less than half of the filling into a separate bowl and stir in the melted chocolate until well blended.
Poor the chocolate filling into the pie shell; gently shake the pan to settle the filling.
Put the pie on the center rack of a 350°F oven; bake for 20 minutes; transfer to a wire rack and let cool 15 minutes.
Gently ladle the remaining plain pumpkin filling over the cooled chocolate layer; shake the pan gently to settle the filling.
Return to the oven, placing the pie so that the part that faced the back of the oven now faces the front; bake 35-40 minutes or until the pumpkin layer is set (When done, the perimeter of the filling will have puffed some but not so much that it develops large cracks).
Transfer pie to a wire rack and let cool until the filling settles down and flattens--about 30-45 minutes.
Combine the sour cream and remaining 1/4 cup sugar in a small saucepan over very low heat, stirring constantly for 2-4 minutes or until warmed.
When the sour cream is fairly thin and slightly warmer than body temperature, carefully pour it over the top of the pie.
Immediately shake and tilt the pie to cover it with the topping.
Return the pie to the rack and let cool completely.
Cover loosely with foil and refrigerate for at least 4 hours.
Note--the chocolate layer is partially prebaked to maintain its integrity when the plain pumpkin layer is added; the prebaking firms up the chocolate and the short cool down allows a slight skin to form, which helps keep the pumpkin layer from sinking into the chocolate.To enhance the breakdown between layers, you might want to sprinkle a fairly dense layer of chopped walnuts or pecans over the chocolate as soon as the pie comes out of the oven, pressing them gently to embed them in the chocolate.