Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the blueberries, granulated sugar, cornstarch, lemon juice, and vanilla extract. Toss gently until the blueberries are coated. Pour the mixture into the greased baking dish.
In another bowl, combine the flour, baking powder, salt, and cinnamon (if using).
In a separate bowl, cream the softened butter and brown sugar together until light and fluffy. Add the egg and mix well.
Gradually add the flour mixture to the butter mixture, stirring until just combined.
Spread the crumbly topping evenly over the blueberry mixture in the baking dish.
Bake in the preheated oven for 40-45 minutes, or until the top is golden brown and the blueberries are bubbling.
Remove from the oven and let cool for a few minutes before serving.