1CRUST: Melt butter in a small saucepan. Stir in graham cracker crumbs, 3 Tbsp. brown sugar, and 1/3 cup chopped nuts. Pack into ungreased 9 or 9-1/2-inch spring form pan.
2FILLING: Beat cream cheese and 1-1/4 cups brown sugar together until smooth and fluffy. Slowly beat in eggs one at a time, just until blended. Stir in 1/2 tsp. rum flavoring, 1 tsp. vanilla, 1/4 cup sour cream, and 1/3 cup chopped nuts. Pour filling into pan.
3Bake in a preheated 350-degree oven for about 55 to 60 minutes, until set. Remove pan to cake rack while you prepare topping.
4TOPPING: Stir together 1-1/2 cups sour cream, 1/4 cup brown sugar, maple flavoring, and 1/2 tsp. rum flavoring. Spread over cheesecake. Return to oven, and bake for 10 minutes. Loosen sides of cake. Cool until almost room temperature, then chill for a few hours before serving.