Preheat oven to 400 degrees. Spray a 9x9-inch glass baking pan with cooking oil spray.
In a large mixing bowl, sift together the flour, oats, sugar, baking powder and salt. Add the milk and egg and stir to combine. Fold in the blueberries.
Pour the mixture into the prepared baking dish.
In a small bowl, sift together the flour and sugar for the topping. Cut the butter into the mixture until coarse crumbs form. Drizzle in the vanilla extract and stir to combine. Sprinkle over the top of the cake.
Bake 30-35 minutes, until the top is golden and the cake is cooked through. Remove from oven and let cool for at least 10 minutes before turning onto a cooling rack.
This can be serve warm or cooled.
Recipe by Maine Course at https://mainecourse.bdnblogs.com/2016/06/07/recipes/blueberry-oatmeal-crumb-cake/