TOPPING: In bowl, combine brown sugar, chopped hazelnuts, flour and cinnamon. Using fork, stir in butter until crumbly; set aside.
In large bowl, beat butter with brown sugar until light and fluffy; beat in eggs, 1 at a time. Beat in vanilla.
In separate bowl, whisk together flour, baking soda, baking powder and salt. Add to butter mixture alternately with buttermilk, making 3 additions of flour mixture and 2 of buttermilk. Fold in chopped rhubarb. Scrape into prepared pan; sprinkle with topping.
Bake in centre of 350°F (180°C) oven until cake tester inserted in centre of cake comes out clean, about 1-1/4 hours. Let cool in pan on rack for 5 minutes. Remove side of pan. Serve warm or cool. Serves 8 to 10.