First make the streusel. Combine pecans, flour, sugar, cinnamon and salt in the bowl of a food processor. Pulse just a couple of times to combine. Add the cold butter and pulse 5 or 6 more times until combined. Tip it out into a bowl and keep refrigerated until needed.
Combine pumpkin puree, brown sugar, eggs, melted butter, vanilla and milk in a large bowl and whisk until evenly combined. Add baking powder, baking soda and salt and whisk to combine.
Sprinkle over the flour and spice and fold it in until just combined. Do not over-mix.
Divide batter evenly among prepared muffin cups, filling them to the rim. Top with a heaped mound of streusel (be generous). Bake for 25 minutes until golden brown and muffins spring back when pressed gently. Transfer pan to a wire rack and let cool for 5 minutes before transferring muffins individually to the rack to finish cooling.