Preheat the oven and prepare the pan

• Preheat the oven to 375°F (190°C).
• Line a muffin pan with paper liners.

Mix the wet ingredients

• In a bowl, mix the pumpkin purée, melted butter, granulated sugar and brown sugar until smooth.
• Add the eggs, vanilla paste and buttermilk and mix until combined.

Fold in the dry ingredients

• Add the flour, baking soda, baking powder, salt, cinnamon and nutmeg.
• Gently fold until just combined, without overmixing.

Make the streusel

• Mix the brown sugar, flour and cinnamon together.
• Work in the softened butter with a fork or your hands until crumbly and pea-sized.
• If the streusel turns into a dough, freeze it for a few minutes; if still too soft, add flour a little at a time and freeze again until crumbly.

Assemble the muffins

• Fill each muffin cup about halfway with pumpkin batter.
• Add a layer of streusel, then cover with more pumpkin batter.
• Generously top each muffin with the remaining streusel.

Bake the muffins

• Bake at 375°F (190°C) for 5 minutes.
• Without opening the oven, reduce the temperature to 350°F (175°C) and continue baking for 18–22 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Cool

• Let the muffins cool in the pan for a few minutes.
• Transfer them to a cooling rack.