Preheat Oven to 350 degrees F.
Melt butter and chocolate in top of a double boiler set over barely simmering water. Remove from heat and whisk until smooth. Set aside.
Using an electric mixer on medium speed, beat cream cheese in a large bowl, gradually adding 1 1/4 cups of the sugar. Beat in eggs, one at a time.
Next, blend in pumpkin, vanilla extract, spices, and salt.
Pour slightly less than half of the filling into a separate bowl and stir in melted chocolate. Pour chocolate filling into prebaked pie shell and gently shake to settle the filling. Bake pie on center oven rack at 350 degrees for 20 minutes. Cool on wire rack for 15 minutes.
Spoon remaining plain pumpkin filling over chocolate layer and shake to settle filling. Return pie to oven and bake approximately 35 to 40 minutes until done. Place pie on cooling rack and cool 30 to 45 minutes.
Combine sour cream and remaining 1/4 cup sugar in small saucepan. Stir mixture over very low heat approximately 2 minutes until fairly thin and slightly warm. Pour evenly over pie. Allow pie to cool, then refrigerate at least 4 hours before serving. Makes 8 to 10 servings.
