Preheat your oven to 190º C and line a 12 hole muffin tin with cupcake liners.
Cream together the butter and sugar in the bowl of a stand mixer until light and fluffy.
Crack the eggs into a jug and add to the mixture, one by one, mixing well.
Add the flour, nutmeg, cinnamon, mixed spice and baking powder. Mix until only just combined, taking care not to overmix.
Add the pumpkin puree (whether homemade or premade) and swirl through the mix.
Distribute the mix between the muffin wrappers. You can use an ice cream scoop for even muffins, or just use a spoon. Whatever works for you! Sprinkle some of the streusel topping on each muffin.
Bake for 8 minutes, then reduce the oven temperature to 175º C and bake for a further 10-12 minutes. The muffins are ready when they're golden on top, and when you insert a toothpick into the muffin it will come out clean.