Prepare the Pumpkin-Walnut Bundt Cake: Preheat oven to 350°. Generously spray a 14-cup bundt pan with baking spray.
Whisk together flour and next 6 ingredients (through salt) in a medium bowl; set aside. In a separate bowl, whisk together pumpkin puree, buttermilk, vanilla extract, and maple extract until smooth.
Beat butter with brown and granulated sugars in a stand mixer fitted with a paddle attachment at high speed until fluffy, at least 5 minutes. Add eggs to bowl, 1 at a time, mixing at medium speed until just combined. Mix flour mixture into butter mixture in 3 additions, alternating with pumpkin mixture in between each addition. Stir in walnuts. Pour batter into prepared bundt pan, spreading evenly with a rubber spatula.
Bake cake at 350° on a rack arranged in the center of the oven for 40 to 50 minutes, or until a skewer inserted in the center of each cake comes out clean. Let cakes cool in pan for 10 minutes. Invert pans, turning cakes out onto wire racks to cool completely.
Prepare the glaze: In a small saucepan over medium heat, melt butter. Whisk in tahini and milk, stirring constantly until well-blended, about 2 minutes. Remove from heat and whisk in powdered sugar and lemon juice. While the cake is still slightly warm, pour glaze over cake. Scoop up any glaze that has dripped off and spoon over cake again. Allow to cool completely before serving.