Adjust oven rack to middle position and heat oven to 325ºF (163ºF). Grease a 9×5-inch loaf pan*. Line pan with a 12-inch x 8-inch piece of parchment paper, making a sling in pan. Fold and crease parchment over edges.
Whisk 2 cups (260g) flour, ¾ teaspoon baking powder and ½ teaspoon salt together; set aside.
Using stand mixer fitted with paddle attachment, beat 16 tablespoons butter and 1 cup (200g) sugar on medium to combine. Increase speed to high and cream until pale and fluffy, 5–7 minutes, scraping down bowl twice throughout process.
Beat in ½ cup (123g) sour cream.
Scrape down bowl and mix in one egg at a time (4 eggs total), followed by 1 tablespoon vanilla.
Scrape down bowl and mix again for about 30 seconds to ensure the eggs are fully incorporated, mixture will look slightly curdled, that’s OK.
With mixer on low, gradually add flour mixture just until incorporated. Use a spatula to give it one final stir.
Spread ⅓ of batter (1 ½ cups; ~338g) into prepared pan, dollop ⅓ of jam (~¼ cup; ~75g) over top and spread into an even layer as best you can, leaving a ½-inch border around pan. Repeat layering with another ⅓ batter and ⅓ jam.
Spread remaining batter over top and remaining jam over top, use a spoon to swirl jam into batter slightly.
Bake until edges are golden brown, cake is tall, and a toothpick inserted in center comes out clean, 1 hour 15 minutes–1 hour 20 minutes, rotating pan halfway through baking.
Let cake cool in pan on wire rack for 10 minutes. Using parchment overhang, lift cake from pan and let cool completely on rack, about 1 hour.