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

Preheat oven to 350 degrees. Line two standard muffin pans, or one double muffin pan with 20 liners. Set aside.

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

Add the melted butter, granulated sugar, eggs, and vanilla to a large bowl. Whisk until smooth and combined.

Add in the pumpkin puree and buttermilk and whisk to combine.

Add in the dry ingredients and whisk until everything is combined and the batter is free of lumps.

Divide the batter between the 20 wells in the muffin pan(s). Top the muffin batter with streusel. Each well will be pretty full.

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

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

Store the leftover muffins in an airtight container for up to three days.