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Your Amish friendship sourdough starter is ready to use once it has doubled in size, has some bubbles, and smells sweet.
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Preheat the oven to 325 degrees F and Grease two 8×4-inch loaf pans or line with parchment paper for easy transfer.
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In a large mixing bowl, whisk together the starter, eggs, peanut butter, oil, sour cream, sugars, milk, and vanilla until smooth.
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In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, salt, and cinnamon (if using). Toss the peanut butter and chocolate chips in the dry mixture to evenly coat them (this helps to prevent sinking).
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Add the dry mixture to the wet mixture and stir until just combined.
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In a small bowl, whisk together the cocoa powder and hot water (or espresso to make a paste. Scoop out about 220 grams (1 cup) of the peanut butter batter and mix it with the cocoa paste.
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Pour only half of the peanut butter batter into the two pans, dividing it evenly. Spoon the chocolate batter over the top of each, then finish by adding the remaining peanut butter batter on top. Gently swirl with a knife for a marbled effect.
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Bake the bread for 55-60 minutes or until a toothpick inserted comes out with a few moist crumbs. Tent with foil if browning too quickly.
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Cool completely in the pan (40-45 minutes) and then transfer the bread to a wire rack.