Preheat your oven to 350°F (175°C). This ensures that your crisp cooks evenly and the topping becomes golden brown.

In a mixing bowl, combine the blackberries, granulated sugar, lemon juice, cornstarch, and vanilla extract. Gently toss to coat the blackberries without mashing them. The sugar will draw out the juices from the blackberries, creating a delicious filling.

Pour the blackberry mixture into an 8-inch square baking dish, spreading it evenly across the bottom.

In another mixing bowl, combine the rolled oats, flour, brown sugar, cinnamon, and salt. Mix well to ensure all dry ingredients are evenly distributed.

Pour the melted butter over the oat mixture and stir with a spatula until the mixture is crumbly and well combined. The butter helps to bind the topping and creates a crispy texture when baked.

Sprinkle the oat mixture evenly over the blackberry filling in the baking dish.

Place the baking dish in the preheated oven and bake for 30-35 minutes, or until the topping is golden brown and the blackberry juices are bubbling around the edges. This indicates that the filling is cooked through.

Once baked, remove from the oven and let it cool for about 10 minutes before serving. This allows the filling to set slightly, making it easier to serve.

Serve warm, optionally topped with vanilla ice cream or whipped cream for an extra treat. The contrast of the warm crisp with cold ice cream is delightful!