Preheat your oven to 350°F (175°C). This ensures even cooking of the bars.

In a mixing bowl, combine the flour, oats, brown sugar, salt, and melted butter. Mix until crumbly.

Press the mixture firmly into the bottom of a greased 9x9 inch baking pan. This forms the base of your bars.

Bake the crust for 15 minutes until lightly golden. This step helps to create a firm base that holds the filling.

In a saucepan over medium heat, combine the blackberries, granulated sugar, cornstarch, lemon juice, and vanilla extract.

Cook for about 5-7 minutes, stirring gently until the blackberries start to break down and the mixture thickens. This creates a delicious, jam-like filling.

Remove from heat and let it cool slightly before spreading over the baked crust.

In a separate bowl, mix the oats, flour, brown sugar, softened butter, and cinnamon until crumbly.

Sprinkle the topping evenly over the blackberry filling. This adds a delightful crunch to the bars.

Return the pan to the oven and bake for an additional 20 minutes, or until the topping is golden brown.

Remove from the oven and allow to cool completely in the pan before cutting into bars. This helps the filling set up properly.