Combine first 3 ingredients; mix well. Firmly press mixture into bottom of a 10-inch springform pan.
Place chocolate squares in top of a double boiler; place over boiling water. Reduce heat to low; cook until chocolate melts. Remove from heat; stir in 2 tablespoons sour cream. Set mixture aside to cool.
Beat cream cheese at medium speed of an electric mixer until light and fluffy; gradually add 1 cup sugar, mixing well. Add eggs, one at a time, beating well after each addition. Add chocolate, remaining sour cream, coffee, and vanilla; mix well.
Pour mixture into prepared pan. Bake at 350° for 45 minutes. (Filling will become firm as pie stands.) Let cool to room temperature on a wire rack; chill 8 hours or overnight.
To serve, remove sides of Springform pan. Garnish with whipped cream, if desired.
Alternate Method: To prepare cheesecake filling using a food processor, position knife blade in food processor bowl; add cream cheese, sugar, and eggs. Process 2 minutes. Stop processor, and scrape sides of bowl with a rubber spatula. Process 1 to 2 additional minutes or until mixture is smooth. Add cooled chocolate mixture, sour cream, coffee, and vanilla; mix well and proceed with recipe.