Soak the Oats: Combine the whole milk and old-fashioned oats in a bowl. Leave the mixture to soak for 20 minutes, allowing the oats to soften before adding them to the batter.
Prepare the Oven and Muffin Pan: Preheat the oven to 425°F. Lightly coat a 12-cup muffin pan with nonstick cooking spray or line each cup with a cupcake liner. Set the prepared pan aside.
Mix the Dry Ingredients: In a large mixing bowl, combine the all-purpose flour, baking powder, baking soda, ground cinnamon, and salt. Stir until the ingredients are evenly distributed.
Combine the Wet Ingredients: In a separate bowl, whisk together the melted unsalted butter, maple syrup, room-temperature egg, and vanilla extract until the mixture is well combined.
Make the Muffin Batter: Add the wet mixture and the milk-soaked oats to the bowl of dry ingredients. Stir gently until the ingredients are just combined. Avoid overmixing to help maintain a tender muffin texture.
Fold in the Blackberries: Add the halved blackberries to the batter. Using a rubber spatula, fold them in gently so they are distributed throughout without being crushed unnecessarily.
Fill the Muffin Pan: Divide the batter evenly among the 12 prepared muffin cups, filling each one to the top. For more even portions, use an ice cream scoop. If desired, sprinkle additional oats and coarse sugar over the tops.
Begin Baking at High Temperature: Place the muffin pan in the preheated oven and bake at 425°F for 5 minutes. This initial high-temperature stage helps the muffins rise.
Reduce the Oven Temperature: Without removing the muffins from the oven, lower the temperature to 350°F. Continue baking for 15–18 minutes, until the muffins are cooked through.
Check for Doneness: Insert a toothpick into the center of a muffin. The muffins are ready when the toothpick comes out clean.
Cool the Muffins: Let the muffins rest in the pan for 5 minutes after baking. Then carefully transfer them to a wire rack to cool further before serving.
