Preheat the oven: Begin by preheating your oven to 375°F (190°C). Prepare a muffin tin by either lining it with paper liners or lightly greasing it with butter to ensure easy removal of the muffins.
Prepare the streusel topping: In a medium-sized mixing bowl, combine the rolled oats, ½ cup of all-purpose flour, ¼ cup of packed brown sugar, and ½ teaspoon of ground cinnamon. Add the cold cubed butter and using your fingers or a pastry cutter, mix until the mixture resembles coarse crumbs with small lumps. Set this mixture aside to allow the flavors to meld.
Mix dry ingredients: In a large mixing bowl, whisk together the 2 cups of all-purpose flour, baking powder, baking soda, salt, 2 teaspoons of ground cinnamon, and ½ teaspoon of ground nutmeg until well combined.
Mix wet ingredients: In a separate medium bowl, whisk together the granulated sugar, packed brown sugar, eggs, vegetable oil, milk, pumpkin puree, and vanilla extract. Mix until everything is smoothly combined and the sugars have dissolved.
Combine wet and dry ingredients: Gently pour the wet ingredient mixture into the bowl with the dry ingredients. Using a spatula, fold the mixtures together gently until just combined – be careful not to overmix. Finally, fold in the diced apples until evenly distributed throughout the batter.
Fill muffin tin: Using a spoon or an ice cream scoop, fill each muffin cup approximately ¾ full with the batter.
Add streusel topping: Generously sprinkle the prepared streusel topping over each filled muffin cup.
Bake: Place the muffin tin in the preheated oven and bake for 18-20 minutes. Check for doneness by inserting a toothpick in the center of a muffin – it should come out clean or with a few crumbs attached.
Cool: Once baked, remove the muffins from the oven and let them cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.
Notes
Serve warm dusted with powdered sugar and pair with whipped cream or apple slices.