Generously butter a muffin tin. Preheat the oven to 375℉.
In a large bowl, whisk together the 2 ½ cups oat flour, 3 teaspoons baking powder, 1 teaspoon ground cinnamon, and ½ teaspoon fine sea salt. Lightly whisk the 1 large egg separately, then stir in the beaten egg, 1 cup whole milk, ½ cup maple syrup, and ¼ cup oil. Gently fold in the 1 cup berries.
Spoon the batter evenly into the muffin tin (fill each cup about two-thirds full). and let the batter rest for up to 20 minutes. Bake the muffins for 15-20 minutes, until a tester comes out clean.
Place the muffin tin on a wire rack to cool for about 10 minutes, then gently turn out the muffins onto the rack to cool completely.
Notes
Rest the batter. Whole-grain flour benefits from a rest before baking for a smoother, softer crumb. You can set a timer for 15 minutes, or let the batter rest while the oven preheats to help it hydrate.
Grease the muffin pan well. It's a good idea to very well grease the muffin tin to help the muffins release easily. Alternatively, use muffin liners.
Fill the muffin cups ⅔ full for this recipe. While some muffin recipes advise filling the muffin cups to the brim, I’ve found that about ⅔ full is the sweet spot so that they rise nicely but don’t overflow.
Try honey. Instead of maple syrup, use honey to sweeten the muffins.