Position an oven rack in the center of the oven, and preheat to 350 degrees F. Line a 12-cup muffin tin with paper liners, then spray the liners with cooking spray. Put the cranberries in a small bowl, and add hot water to cover (if the cranberries are very moist, you can skip this step).
Whisk together the flour, oats, baking powder, salt and baking soda in a medium bowl; whisk in the chocolate chips. Whisk together the brown sugar and sunflower butter in a second medium bowl, then whisk in the milk, oil and egg. Stir the sunflower butter mixture into the flour mixture to combine well (the batter will be pretty loose). Strain the cranberries, pat them dry and stir them into the batter.
Ladle 1/3 cup batter into each muffin cup liner. Bake, rotating the pan about halfway through, until a cake tester inserted in a few muffins comes out clean, 16 to 18 minutes.
Let cool briefly in the tin on a rack, then transfer the muffins to the rack to cool completely. The muffins can be stored overnight in an airtight container, or individually wrapped in plastic and frozen in an airtight container for up to 4 days.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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