Preheat oven to 500F.

Stir together butter and mint extract (if used). Stir in crumbs until well blended. Press crumb mixture evenly over bottom and about one half inch up sides of greased 9-inch spring form pan. Set aside.

In a large mixing bowl, beat together cream cheese, chocolate, 1 1/3 cups sugar, flour, and one teaspoon vanilla at low speed until blended. Beat at high speed until smooth. Add eggs, one at a time, beating well after each addition. Pour into prepared pan. Bake at 500F for 10 minutes. Reduce heat to 250F and continue baking until cake tester inserted near center comes out clean, an additional 50 to 60 minutes. Cool on wire rack 10 minutes. Leave oven on.

Blend together sour cream, 4 tablespoons sugar and 1 teaspoon vanilla. Spread evenly over cake. Bake an additional 5 minutes. Cool on wire rack 45 minutes to one hour.

Refrigerate until firm, at least 8 hours or overnight. Garnish with chocolate curls.