Preheat your oven to 375 F. Line two standard muffin pans with liners and set them aside.
In a small bowl, combine ¾ cup flour, ⅔ cup brown sugar, ½ teaspoon pumpkin pie spice, ½ teaspoon salt, and butter, then mix everything with your fingertips until uniform. Toss in walnuts; set aside.
Stir or whisk to combine 3 cups flour, 2 cups brown sugar, baking powder, baking soda, 4 ½ teaspoons pumpkin pie spice, and 1 ½ teaspoons salt together in a medium-sized bowl.
Add the pumpkin, eggs, buttermilk, and oil to the bowl, and stir until the batter is smooth. Stir chocolate chips into the batter.
Use a scoop or spoon to divide batter evenly between muffin cups. Top each muffin evenly with streusel.
Bake the muffins 28-30 minutes, or until they are golden brown and spring back to the touch. Transfer the muffins to a wire rack to cool.
Stir confectioners' sugar, milk, and maple extract together to form a smooth glaze. Drizzle the glaze over the cooled muffins.