PREHEAT oven to 350° F.
COMBINE crumbs, butter and sugar in medium bowl. Press onto bottom and 1-inch up side of ungreased 9-inch springform pan.
BAKE for 8 to 12 minutes or until golden brown. Cool completely in pan on wire rack.
MELT morsels in 2-quart saucepan over low heat for 5 to 6 minutes, stirring occasionally, until melted. Cool for 15 minutes.
COMBINE cream cheese and 3/4 cup sugar in large mixer bowl. Beat at medium speed for 2 to 3 minutes, scraping bowl often. Gradually add eggs, one at a time, beating well after each addition. Reduce speed to low; add melted chocolate, 1/2 cup sour cream and 2 teaspoons vanilla extract. Beat until well mixed for 1 to 2 minutes. Pour cheesecake mixture into baked crust.
BAKE for 1 hour 20 to 1 hour 35 minutes or until just set 2 inches from edge of pan.
COMBINE 1 1/2 cups sour cream, 2 tablespoons sugar and 1 teaspoon vanilla extract in small bowl. Spread evenly over top of hot, partially baked cheesecake. Continue baking for 5 minutes. Let stand uncovered until cooled (1 to 2 hours). Loosen sides of cheesecake by running knife around inside of pan. Loosely cover; refrigerate 8 hours or overnight. Store refrigerated.