In a bowl, whisk the flour, cinnamon, nutmeg, baking powder, baking soda, and salt. Set aside.
In another bowl, whisk the sour cream, oil, jam, eggs, sugars, and vanilla until homogeneous.
Add the dry mix to the wet batter and mix until no dry patches remain.
Transfer the batter to a parchment-lined baking dish or springform. (If you don't want to use parchment paper, you can brush the bottom and sides of the dish with soft butter or oil)
Bake the cake at 350F (180C) on the middle rack for about 20 minutes; start checking after 18 minutes. When a toothpick comes out clean from the middle of the cake, it's ready. Take the cake out of the oven and let it cool.
Once the cake has cooled to room temperature, take a serrated knife and cut off the dome of the cake so you have a flat surface.
Spread the pumpkin jam over the top.