Preheat oven to 350°F (175°C). Line a muffin tin with paper liners or grease lightly.
Prepare streusel: mix cold butter, brown sugar, flour, and salt until crumbly. Set aside.
In a bowl, whisk together flour, baking soda, pumpkin pie spice, and salt.
In another bowl, beat melted butter and sugar until combined. Add eggs, vanilla, and pumpkin puree.
Gradually mix dry ingredients into wet, alternating with buttermilk. Do not overmix.
Fill muffin cups two-thirds full. Top with streusel mixture.
Bake for 18–20 minutes, until a toothpick inserted comes out clean.
Cool 5 minutes in tin, then transfer to a wire rack. Serve warm or at room temperature.
Notes
You can add chocolate chips, nuts, or swap in gluten-free flour for dietary adjustments. Store in an airtight container for 2–3 days or freeze up to 3 months.