Preheat the oven to 350 degrees F and grease a loaf pan with cooking spray.
Add all of the ingredients, aside from berries in a large bowl and mix to combine.
Melt the frozen berries on the stovetop in a small saucepan over low heat for 4-5 minutes or in the microwave for about 1 minute. Use a fork or potato masher to crush the berries into a jam-like texture.
Pour HALF of the batter into the greased loaf pan.
Next, pour HALF of the berries over the batter and swirl/marble, using a fork or knife.
Repeat steps 4 and 5 with the remaining batter and berries.
Bake for 45-50 minutes or until a toothpick comes out clean.
Let the loaf cool for about 30 minutes, slice and enjoy!
Video
Notes
*Add a ¼ teaspoon of salt if you are using unsalted peanut butter.**Replace frozen berries with your favorite homemade or store-bought jam.