Preheat oven to 350°F. Spray muffin pans with nonstick cooking spray or line with muffin liners.: The warm air of a properly heated oven is what initiates lift and sets the muffin structure immediately, so you should smell a neutral, slightly dry warmth when the oven reaches temperature. Preheating ensures the batter meets consistent heat right away, promoting even doming and proper rise. A common mistake is placing batter into a cool oven which leads to flat, dense muffins. Also, whether you use liners or spray, make sure pans are clean so muffins release easily.
Make the topping by stirring together all topping ingredients with a fork in a medium sized bowl. Set aside.: As you work the butter into the flour and sugars, you will hear a soft scraping and notice the mixture breaking into small clumps, the look you want for streusel. The aromas of brown sugar will become more evident as it mixes with warm butter, hinting at caramelized notes. If you overwork the mixture it can become pasty instead of crumbly, so stop once small clusters form. Left to sit, the streusel holds its shape better on the batter when applied right before baking.
Stir (or mix with hand mixer) together granulated sugar, brown sugar, pumpkin puree, and oil until smooth. Stir in salt, baking soda, pumpkin pie spice, and eggs.: Right away you will notice a silky, glossy batter as the sugars dissolve into the pumpkin puree and oil , releasing warm, spicy aromas from the pumpkin pie spice . The sound of a hand mixer will be a steady whir, and the texture should be homogenous with no pockets of sugar. This step builds the flavor base and ensures these wet components are fully integrated before adding dry flour, which prevents overmixing. A frequent error is under mixing here, leaving streaks of unmixed sugar or lumps of puree, so scrape the bowl sides thoroughly.
Add vinegar, milk, and flour and stir or mix until no lumps remain.: As you fold in the milk and flour , the batter will change from loose to a thicker, ribbon like consistency, and you may catch a faint tang from the vinegar reacting with the baking soda . That reaction creates tiny bubbles that help the muffins rise. The batter should be smooth, but do not overwork it as that develops gluten and yields tougher muffins. A typical mistake is vigorous mixing at this stage, which can lead to dense, gluey texture; instead fold just until combined and no dry streaks remain.
Fill muffin cups about 2/3 full. Sprinkle each muffin with streusel topping. Bake for 15-22 minutes, or until a toothpick comes out clean from the center of the muffin. Cool slightly before eating.: When you fill cups two thirds full you will allow room for the muffins to dome without spilling over. As they bake, you will hear a quiet settling and notice the tops changing color to a warm, golden brown while the streusel crisps. A clean toothpick pulled from the center signals doneness, and the interior will be moist but set; underbaked muffins will feel jiggly and may leave batter on the tester. Avoid opening the oven door frequently, as that causes temperature fluctuation and may prevent proper rise. Letting muffins cool briefly helps the crumb stabilize so they do not crumble when removed from the pan.
