Preheat oven to 400°F. Line a standard 12-cup muffin pan with paper liners.
Halve and pit the plums. Thinly slice 2 plums for garnish. (You’ll need 2 or 3 slices for each muffin.) Cut the remaining plums into a small dice (about ½ cup).
In a medium bowl, whisk together the hazelnut meal, oat flour, baking powder, salt, cinnamon and baking soda.
In a large bowl, whisk together the egg yolks, brown sugar, vanilla, and buttermilk. Stir in the flour mixture. Fold in the diced plum.
In a medium bowl, with a clean whisk, beat the egg whites until stiff peaks form, about 2 minutes. Stir ⅓ of the egg whites into the batter, then carefully fold in the remainder.
Divide the batter evenly between the muffin cups. Top each cup with a few slices of plum and sprinkle with granulated sugar. Transfer the pan to the oven and immediately reduce the temperature to 350°. Bake until the muffins spring back when pressed gently, 18 to 22 minutes. Let muffins cool in the pan for 5 minutes then transfer them to a rack to cool completely.