To a small bowl, add your melted butter and sugar and stir together. Add your flour and salt and stir again till combined (the mixture will be clumpy). Put in the fridge while you make the rest of the muffins.
Preheat your oven to 425 and line a cupcake pan with liners (or spray with non-stick spray).
To a large mixing bowl, add your butter, white sugar and brown sugar and beat together with a mixer till creamy. Add your pumpkin puree and vanilla and mix together till combined. Add your eggs and briefly mix them in.
To the same bowl, add your flour, baking powder, baking soda, pumpkin pie spice and salt. Stir together till you have a thick, smooth, consistent muffin batter.
Fill each cupcake liner about 3/4 full with batter. Spread the batter of each muffin, so they're flat across the top (this will stop the streusel topping from falling down the sides).
Remove your streusel topping from the fridge and sprinkle it across the top of each muffin (about 1 tbsp or so per muffin).
Bake in the oven for 5 minutes. Reduce the oven temperature to 350, and bake for another 14-16 minutes (or till the muffins are springy to touch and a toothpick inserted comes out clean).
Stored in an airtight container at room temperature these will last about 1-2 days. In the fridge, they will last about a week.