1. Make cake: Preheat oven to 350ºF. Butter and flour a 10-cup Bundt pan.
2. Sift flour, baking powder, baking soda, spices and salt together in a bowl. In another bowl, whisk together pumpkin, buttermilk and vanilla. In a large bowl, using an electric mixer on medium-high speed, beat butter and brown sugar until light and fluffy, about 4 minutes. Beat in eggs, one at a time, beating well after each. Scrape down sides and bottom of bowl. Beat in one third of flour mixture, then half of pumpkin mixture, beating until just combined. Repeat, ending with last third of flour mixture. Scrape batter into Bundt pan, smoothing top. Bake until a toothpick inserted in center of cake comes out clean, 45 to 50 minutes.
3. Let cake cool in pan on a wire rack for 15 minutes, then turn out onto rack to cool completely.
4. Make glaze: Whisk together milk and maple syrup and extract. Whisk in confectioners' sugar until glaze is smooth and of drizzling consistency. Up to 6 hours before serving, set cake on a wire rack over a plate and drizzle with glaze, allowing it to run down sides. Let cake rest at room temperature to allow glaze to set.