• Preheat oven to 350 degrees F. Prepare your bundt pan by greasing it with melted butter and dusting with flour, or use a baking spray that contains flour to evenly coat the pan.

Streusel

• In a small bowl, combine walnuts or pecans, brown sugar, and cinnamon. Use a spoon to blend until all ingredients are thoroughly incorporated.

Batter

• In a medium bowl, whisk the flour, baking powder, baking soda, salt, and spices.
• In a separate large bowl, beat the pumpkin puree, sugars, oil, buttermilk, eggs, maple syrup, and vanilla until combined.
• Add the flour mixture to the wet ingredients. Use a large rubber spatula or wooden spoon to mix until there are no more large streaks of flour (a few lumps are ok). Do not over-mix the batter.
• Spoon half of the batter into the prepared bundt pan. Top with the streusel mixture, then cover with the remaining batter. Tap the pan several times on the counter top or a cutting board to release any air bubbles. 
• Bake for 1 hour 15 minutes, or until a wooden skewer inserted into the center of the cake comes out clean and the side are beginning to pull away from the pan. Transfer to a wire rack and let cool for 15 minutes before inverting pan to remove cake.

Glaze

• To make the glaze, whisk together the confectioner's sugar and 1 tablespoon milk and vanilla. Add additional 1 teaspoon of milk at a time to reach desired consistency, then pour over cooled cake.