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Preheat oven to 350°F. For the Crumb Topping, mix walnuts, flour, brown sugar and cinnamon in medium bowl. Cut in butter with pastry blender or 2 knives until mixture resembles coarse crumbs.
For the Cake, mix flour, cream of tartar, baking soda and salt in small bowl. Set aside. Beat butter, granulated sugar and vanilla in large bowl with electric mixer on medium speed until light and fluffy. Beat in eggs and sour cream on low speed just until blended. Gradually beat in flour mixture until blended. Stir in pears. Spread evenly in greased 13x9-inch baking pan. Sprinkle with Crumb Topping.
Bake 45 to 50 minutes or until cake pulls away from sides of pan and topping is browned. Cool in pan on wire rack.