Preheat the oven to 350°F Grease your 10" bundt or fluted pan and set aside.
Prepare the nut filling by combining the flour and butter with a fork until you have a crumbly mixture. Mix in the cinnamon, chopped nuts and maple syrup to form a lumpy paste, then set aside.
For the cake, stir together the flour, baking powder and baking soda. In a separate bowl, cream the butter and sugar together until light and fluffy. Beat in one tablespoon of the flour mixture then one egg, then another tablespoon of flour mixture, then the final egg. Add the remaining flour and and beat until well combined, then mix in the sour cream. The batter should be thick.
Spoon just more than half the cake batter into the bundt pan. Spread the mixture up the sides a bit and around the funnel of the pan to create a rim to keep the filling inches.
Carefully drop in the maple nut filling into the dent in the cake batter, then cover the filling with the remaing cake batter. Smooth the top of the cake and place in the middle of a preheated oven.
Bake the cake for 30 - 35 minutes or until a cake tester inserted in the cake comes out without batter (note: the filling may stick to the tester).
Let the cake cool on the rack for 15 minutes, then loosen the edges and turn the cake onto the rack to cool completely.
Dust the cooled cake with icing sugar just before serving.