Remove the egg, sour cream, buttermilk, and cream cheese from the refrigerator and allow them to come to room temperature.

Preheat the oven to 425°F (220°C). Line two standard muffin pans with 12 liners total, spacing the liners apart when possible.

Spread the pumpkin puree over a plate. Press paper towels gently over the pumpkin 3–4 times to absorb excess moisture.

In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, pumpkin pie spice, and nutmeg.

In a separate bowl, combine the blotted pumpkin puree, melted and cooled butter, brown sugar, egg, vanilla, sour cream, and buttermilk. Stir until smooth and evenly combined.

Pour the wet ingredients into the dry ingredients. Fold gently with a rubber spatula just until combined. The batter should remain thick, and a few small lumps are fine.

Divide the batter among the prepared muffin liners, filling each almost to the top. Sprinkle each muffin with a little coarse sugar.

Make the filling by beating the softened cream cheese, granulated sugar, flour, milk, and vanilla until smooth and creamy.

Transfer the filling to a piping bag. Insert the tip about one-third of the way into each portion of muffin batter and squeeze some cream cheese mixture into the center.

Bake the Pumpkin Cream Cheese Muffins at 425°F for 8 minutes. Without opening the oven unnecessarily, reduce the temperature to 350°F (175°C) and continue baking for approximately 6–8 minutes.

Check the cake portion of a muffin with a toothpick, avoiding the cream cheese center. The toothpick should come out clean or with a few moist crumbs.

Allow the muffins to rest in the pan for 10 minutes before transferring them to a wire rack to cool.