Preheat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line with parchment (leave overhang for easy lifting), if desired.
Make the cream cheese swirl: in a small bowl, beat the softened cream cheese with sugar, egg yolk, and vanilla until smooth. Set aside.
In a medium bowl, whisk the dry ingredients: flour, baking soda, baking powder, pumpkin pie spice, cinnamon, nutmeg, and salt.
In a large bowl, whisk the wet ingredients: pumpkin puree, eggs, vegetable oil, sour cream, and vanilla until fully combined.
Add the dry ingredients to the wet ingredients and fold just until no dry streaks remain (do not overmix).
Assemble: spread half of the pumpkin batter in the prepared pan. Spoon the cream cheese mixture over the batter in an even layer, then top with the remaining pumpkin batter.
Swirl gently with a butter knife or skewer, making a few figure-eight passes for a marbled look (avoid over-swirling).
(Optional) Make streusel topping: in a small bowl, combine flour, brown sugar, and cinnamon. Cut in the cold butter with a fork until coarse crumbs form. Sprinkle evenly over the batter.
Bake for 60–70 minutes, or until the top is golden and a toothpick inserted into the pumpkin portion comes out mostly clean (a little moisture near the cream cheese is okay).
Cool in the pan for 15 minutes, then lift out (or carefully turn out) and cool on a wire rack for at least 15 minutes more before slicing.