In a medium bowl, combine cold unsalted butter, brown sugar, all-purpose flour, and a pinch of salt. Use your fingers or a pastry cutter to mix until you achieve large clumps. Cover and refrigerate this mixture.

Preheat your oven to 350°F (175°C). Line two muffin pans with 20 muffin liners.

In a small bowl, whisk together the all-purpose flour, baking soda, salt, and pumpkin pie spice until well combined.

In a large bowl, blend melted unsalted butter with granulated white sugar, eggs, and vanilla extract until smooth. Gradually stir in pumpkin puree and buttermilk.

Gently fold the dry ingredients into the wet mixture until just combined, ensuring no lumps remain.

Divide the pumpkin batter among the prepared muffin liners, filling each about three-quarters full. Sprinkle the streusel topping over the batter and press down lightly.

Bake for 18–20 minutes, checking for doneness with a toothpick. Muffins should rise and have a golden-brown top.

Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire cooling rack.