Add the cold butter, brown sugar, flour, and salt to a large bowl. Using your hands, mash everything together until combined, forming large clumps. Store the streusel in the fridge until ready to use.

Preheat oven to 350 degrees. Line two standard muffin pans with 20 liners. Set aside.

In a small bowl, add the flour, baking soda, salt, and pumpkin pie spice. Whisk until combined.

In a large bowl, add the melted butter, granulated sugar, eggs, and vanilla. Whisk until smooth.

Add the pumpkin puree and buttermilk, whisking until combined.

Add the dry ingredients and whisk until the batter is smooth and free of lumps.

Divide the batter between the 20 muffin wells and top with streusel.

Bake for 18-20 minutes or until a toothpick comes out clean from the centers with a few moist crumbs.

Let the muffins cool in the pan for five minutes, then transfer to a cooling rack to cool completely.

Store leftovers in an airtight container for up to three days.