Preheat oven to 350°F and spray a 9x5-inch loaf pan with baking spray. Line pan with parchment paper, leaving an overhang to help with removal of the loaf.
In a medium bowl, whisk together flour, pumpkin pie spice, baking soda, and salt. Set aside.
In a large bowl, whisk together eggs and brown sugar until smooth. Add in vanilla, melted butter, pumpkin, and sour cream, and whisk well until smooth.
Using a rubber spatula, gently fold in dry ingredients until they are just barely combined.
Transfer to prepared loaf pan and bake in preheated oven for about an hour, or until a toothpick entered into the center of the loaf comes out clean. Cool completely on a wire rack.
Maple Brown Butter Glaze
Heat a light-colored sauté pan over medium-high heat. Add cubed butter and melt, whisking regularly. Butter will start to foam and milk solids will begin to separate. Your brown butter is done when milk solides are a deep caramel color and the butter gives off a nutty aroma. Immediately remove from pan to a heatproof container, scraping brown bits along with it. Let cool.
Stir together cooled brown butter, maple syrup, and vanilla.
Sift in powdered sugar and whisk until smooth. Pour over cooled pumpkin bread.
Pumpkin Pie Spice Alternative: If you don't have store-bought pumpkin pie spice blend, you can make your own! Just use 1½ tsp cinnamon, ¾ tsp ginger, ½ tsp nutmeg, and ¼ tsp allspice.Brown Butter: For more details and a step-by-step on browning butter, check out How to Brown Butter, a.k.a. Brown Butter Appreciation Post.