Preheat the oven to 350°F (175°C) and grease a muffin tin.

In a mixing bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt.

In another bowl, combine the pumpkin puree, diced apple, brown sugar, granulated sugar, egg, and milk.

Gradually add the dry ingredients to the wet ingredients and mix until just combined.

In a small bowl, mix together the flour, brown sugar, cinnamon, and cubed butter.

Use a fork or your fingers to combine until crumbly.

Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full.

Sprinkle the streusel topping evenly over each muffin.

Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean.

Allow to cool in the pan for 5 minutes before transferring to a wire rack.