Preheat oven to 375 degrees F.
Finely dice the washed apples. Set aside.
In a large mixing bowl, or bowl of a standing mixer, combine sour cream, brown sugar, eggs, and vanilla until smooth.
In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
Add flour mixture to sour cream mixture in two increments, mixing well between additions. Make sure to scrape the sides and bottom of the bowl.
Gently fold in apples and half of the walnuts by hand with a large spatula.
Divide into prepared bread pans. (I use Baker's Joy to prepare my baking pans.)
Large pan (9 x 5 in.) = 50-60 minutes
Small pans (3 x 5.5 in.) = 25-30 minutes
Halfway through baking time, top loaves with remaining walnuts.
Baking times are approximate. Loaves are done when a knife inserted in the center comes out clean.
Cool in pans for 5 minutes and then remove. Slice when completely cool.