Preheat your oven to 375°F (190°C) and lightly grease a baking dish.
Gently toss the blueberries with 3 tablespoons of sugar and lemon juice in a mixing bowl until evenly coated. Transfer the berry mixture into your prepared baking dish, spreading it out evenly.
In a separate bowl, combine the flour, cinnamon (if using), and 1/4 cup of sugar for the topping.
Add the cold, cubed butter to the dry ingredients. Use a pastry cutter or fork to cut the butter into the flour mixture, working quickly until the mixture resembles coarse crumbs with some larger clumps.
Sprinkle the crumbly streusel evenly over the blueberries in the baking dish, covering the surface completely.
Place the dish in the oven and bake until the topping is golden brown and the filling is bubbling around the edges, about 40 minutes. The aroma of baked berries and butter will fill your kitchen.
Remove the cobbler from the oven and let it cool slightly. Serve warm, ideally with a scoop of vanilla ice cream or a dollop of whipped cream for extra indulgence.