Preheat oven to 350°F (180°C). Grease 12 large muffin tins. Set aside.
In small bowl, whisk together flour, ground walnuts, baking soda, baking powder and salt.
In large bowl, beat together butter and sugar with an electric mixeruntil pale and fluffy, about 3 minutes. Beat in one egg at a time,beating well after each addition. Beat in sour cream and maple extractuntil smooth and fluffy, about 4 minutes (2 minutes with a standingmixer). Reduce speed to low and add flour mixture, scraping down sidesof bowl and mixing just to combine.
In food processor, pulse together walnuts, cinnamon, gingerand sugar until nuts are chopped. Drizzle in butter and pulse once justto combine.
Spoon half of the batter into the muffin tins. Sprinkle with half thewalnut streusel. Spoon in remaining batter and top with streusel. Bakein the centre of the oven until cakes begin to pull away from the sidesof the tin, the tops are golden and set, and a tester comes out clean,about 25 to 30 minutes.
Cool maple walnut coffee cakes on rack in pan for 1 hour. Run a thin knife around edges toloosen and transfer to rack to cool completely or serve warm sprinkledwith icing sugar.
Store in an airtight container for up to 2 days.
Tip: Alternatively, this recipe can be made into one bundt cake and then baked 45 to 60 minutes.
Per one muffin serving, about: 420 calories, 5 g protein, 24 g fat (12 g sat. fat), 46 g carbohydrates, 1 g fibre, 85 mg cholesterol, 240 mg sodium