In bowl of stand mixer, cream butter and sugars together until light and fluffy.
Add eggs and vanilla, one at a time, beating briefly in between each.
Add sour cream. Mix briefly. Add peanut butter and mix until smooth.
Sprinkle baking powder and baking soda over top of wet ingredients.
Add flour and buttermilk in alternating amounts: 1 c. flour, 1/2 c. buttermilk, 1 c. flour, 1/2 c. buttermilk.
Heat jam or preserves in microwavable bowl for 20-30 seconds, just to make it a little easier to spread.
Pour 1/2 batter into 2 greased loaf pans. Spread 1/2 c. jam on top of batter using a knife.
Pour rest of batter on top. Using a spoon, drop dollops of remaining jam on top of bread. Use a knife to swirl jam in a back-and-forth pattern.
Bake at 350 for 45-55 minutes, or until toothpick inserted comes out clean (there may be jam on toothpick, but no batter).
Let cool before slicing. For over-the-top flavor, slather MORE peanut butter on each slice before eating.