PreparationMake crust: Combine all ingredients, then press onto bottom of springform pan. Make filling and bake cheesecake: Preheat oven to 325°F with rack in middle. Melt chocolate with butter, then remove from heat and whisk in cream cheese until smooth. Whisk in sour cream and vanilla. Whisk together eggs and sugar in a large bowl until mixture has a mousse-like consistency, then stir in chocolate mixture and nuts. Pour filling into crust and bake 1 1/2 hours. (Top will be slightly cracked.) Cool to room temperature in pan on a rack, about 1 hour, then chill at least 1 hour. (Cake will sink slightly.) Cooks' notes:•Wheatmeal biscuits are British-style whole-wheat crackers. Look for Carr's (labeled "Whole Wheat Crackers" and found at most supermarkets) or McVitie's brand (found at some specialty foods shops). •Chocolate hazelnut cheesecake can be chilled up to 1 day. Bring to room temperature before serving.