On the stove or in the microwave, melt the butter. Let cool slightly, then place in a medium bowl and whisk in the milk, egg, yolk and vanilla.
In a large bowl, whisk together the oat flour, sugar, baking powder and salt. Add the butter mixture all at one and stir gently to combine. Very gently fold in the blackberries.
Divide the batter evenly between the muffin cups. Let the batter rest while the oven preheats, or about 10 minutes.
Meanwhile, make the topping if using: in a small bowl, combine all of the ingredients and rub together with your fingers until crumbly. Sprinkle the topping evenly over the tops of the muffins.
Place in the oven and bake for 20-30 minutes, until the muffins are set and a tester comes out clean. Remove from the oven and turn the muffins out onto a rack to cool. They will be quite soft, so let them cool at least 30 minutes before eating.