Step 1: In a large saucepan or Dutch oven, combine the fresh blackberries, granulated sugar, fresh lemon juice, and salt. Stir gently to combine.
Step 2: Place the saucepan over medium heat. Cook, stirring occasionally, until the berries begin to release their juices and the mixture comes to a gentle simmer, about 5-7 minutes. Avoid mashing the berries too much if you prefer a chunkier filling.
Step 3: While the berries are simmering, prepare the cornstarch slurry: In a small bowl, whisk together the cornstarch and 1/4 cup cold water until smooth and lump-free.
Step 4: Once the blackberry mixture is simmering, slowly pour the cornstarch slurry into the saucepan while continuously stirring. Continue to cook, stirring constantly, for another 2-3 minutes, or until the filling thickens and becomes glossy. It should be thick enough to coat the back of a spoon.
Step 5: Remove the saucepan from the heat. Stir in the unsalted butter and vanilla extract until they are fully incorporated and melted.
Step 6: Transfer the hot blackberry pie filling to a heatproof bowl or container. Place a piece of plastic wrap directly on the surface of the filling to prevent a skin from forming. Let it cool completely at room temperature, then chill in the refrigerator for at least 2-3 hours (or overnight) to allow it to set fully before using.