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1
Make the crumb mixture: In a food processor blend together the flour, the walnuts, the brown sugar, the butter, the cinnamon, and the salt until the nuts are ground and the mixture is crumbly, add the syrup, and pulse the motor until the mixture is combined well.
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2
Preheat the oven to 350 F. and butter and flour lightly a 9-inch pan.
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3
Make the batter: Into a bowl sift together the flour, the baking powder, the baking soda, and the salt. In another bowl with an electric mixer cream the butter with the brown sugar until the mixture is light and fluffy, beat in the maple syrup, the maple extract, and the vanilla, and beat in the eggs, 1 at a time, beating well after each addition. Add the flour mixture to the butter mixture alternately with the sour cream, beginning and ending with the flour mixture and beating the batter after each addi
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4
Spoon half the batter into the pan, spreading it evenly, sprinkle half the crumb mixture over it, and spread the remaining batter evenly over the crumb mixture. Sprinkle the remaining crumb mixture over the top and bake the cake in the middle of the oven for 50 to 60 minutes, or until a tester comes out clean. Let the cake cool in the pan on a rack for 5 minutes, remove the side of the pan, and transfer the cake, removing the bottom of the pan, to the rack. Let the cake cool completely and sprinkle the top with the confectioners' sugar. The coffeecake may be made 1 day in advance and kept wrapped in plastic wrap.
