Preheat the oven to 450°F. Spray a jumbo muffin tin thoroughly with nonstick cooking spray.
Make the streusel. Combine the flour, sugar, and cinnamon. Stir in the melted butter until the mixture looks like wet sand and holds together when squeezed. Add pecans or oats if desired.
Whisk the flour, baking soda, baking powder, salt, and pumpkin pie spice together in a medium bowl.
In a large bowl, whisk together the pumpkin puree, oil, milk, granulated sugar, brown sugar, egg, and vanilla until smooth.
Add the dry ingredients to the wet ingredients and fold gently just until combined.
Divide the batter between six jumbo muffin cups, filling each all the way to the top. When possible, fill every other cup so there is space between the muffins.
Cover each muffin generously with streusel, lightly pressing some of the larger crumbs into the batter.
Bake at 450°F for 10 minutes.
Without opening the oven door, reduce the temperature to 350°F and continue baking for about 15 minutes.
Check the centers with a toothpick. Continue baking if necessary until it comes out clean or with a few moist crumbs.
Cool in the pan for about 10 minutes, then transfer to a wire rack.