Preheat oven to 350 degrees F. Butter a 10-inch springform pan; set aside. In a food processor or blender combine the 1 1/2 cups walnut pieces and the 1/4 cup granulated sugar; cover and pulse or blend with several on/off turns until fine crumbs form. Add the 1/4 cup melted butter; pulse or blend to mix. Press crumb mixture firmly into the bottom of the prepared springform pan. Bake for 8 to 10 minutes or until crust is firm and lightly browned on the edges. Cool in pan on a wire rack.
Meanwhile, in a large bowl combine cream cheese, mascarpone cheese, the 1/3 cup granulated sugar, and the brown sugar. Beat with an electric mixer on medium-high speed about 5 minutes or until very light and fluffy. Scrape down sides of bowl; beat for 1 minute more. Add eggs, one at a time, beating well after each addition. Stir in the 1 teaspoon vanilla, the maple flavoring, and the 1/4 teaspoon salt; mix well.
Pour cheese mixture into cooled crust. Bake in the 350 degrees F oven for 25 to 30 minutes or until cheesecake is puffy around the edges but wiggles slightly when gently shaken. Cool in pan on a wire rack for 20 minutes.
In a small bowl combine yogurt, maple syrup, the 1/2 teaspoon vanilla, and the 1/8 teaspoon salt. Spread yogurt mixture evenly over cheesecake, spreading to within 1/2 inch of edges. Bake in the 350 degrees F oven for 10 minutes more. Remove cheesecake from oven; while hot, run a thin knife around the edges. Cool in pan on a wire rack. Chill for at least 8 hours or up to 5 days before serving.
To serve, loosen cheesecake from sides of pan; remove sides of pan. If desired, garnish cheesecake with candied walnuts.