In a large mixing bowl, combine cold unsalted butter, brown sugar, all-purpose flour, and a pinch of salt. Use a fork or your fingers to mash the mixture until large clumps form. Cover and refrigerate for at least 15 minutes.

In a separate bowl, whisk together the all-purpose flour, baking soda, salt, and pumpkin pie spice. Set aside.

In a large mixing bowl, melt the unsalted butter and let it cool slightly. Whisk in granulated sugar, eggs, and vanilla extract. Then add the canned pumpkin puree and buttermilk, stirring well.

Gently fold the dry ingredients into the wet mixture until just combined; the batter will be lumpy.

Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners. Fill each muffin cup about two-thirds full with the batter and sprinkle streusel topping generously. Bake for 18-20 minutes.

Let the muffins cool in the pan for 5 minutes before transferring to wire racks to cool completely.