Step 1: Grease 9-inch (2.5 L) spring-form pan; set aside.
Step 2: TOPPING: In bowl, combine brown sugar, chopped hazelnuts, flour and cinnamon. Using fork, stir in butter until crumbly; set aside.
Step 3: In large bowl, beat butter with brown sugar until light and fluffy; beat in eggs, 1 at a time. Beat in vanilla.
Step 4: 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.
Step 5: Bake in center of 350°F (180°C) oven until cake tester inserted in center of cake comes out clean, about 1-1/4 hours. Let cool in pan on rack for 5 minutes. Remove side of pan. Drizzle with maple syrup.
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