Preheat oven to 375 F (190 C). Place 12 muffin liners in a standard muffin pan.
Streusel Topping: In a small bowl, whisk the flour, cornstarch, sugar, cinnamon, and salt. Slowly drizzle in the melted butter and combine gently with your fingers until crumbles form. Set aside
Dry Ingredients: In a large bowl, add flour, cornstarch, baking powder, pumpkin spice, and salt. Whisk to combine well and set aside.
Wet Ingredients: In a medium bowl, add the pumpkin puree, oil, sugar, brown sugar, applesauce, and vanilla. Whisk to combine well.
Now add the wet ingredients into the large bowl of dry ingredients. Fold batter gently with a spatula until just combined. The batter should be thick. Don't overwork the batter or it can cause dense and chewy muffins that don't rise well.
Divide batter evenly into each slot about 3/4 full (an ice cream scoop works great for this). Sprinkle crumble on top and lightly press on top of batter so it sticks (don't push in).
Bake them in the oven for 18-22 minutes or if a toothpick inserted in the middle comes out clean. Don't over bake muffins or they will be dry. Remove from oven.
Let cool for 10 minutes in the pan before transferring them to a cooling rack for another 10-15 minutes.