Preheat your oven to 375°F (190°C) and prepare a 9x13 inch baking dish by lightly spraying it with nonstick spray.

Crush the pretzels using a food processor or a rolling pin in a sealed plastic bag until small bits form.

Combine crushed pretzels, brown sugar, all-purpose flour, and vanilla extract in a mixing bowl. Stir in the melted butter until well mixed.

Spread the pretzel mixture in the prepared baking dish, pressing down firmly. Reserve ½ cup for topping. Bake for about 10 minutes, then cool completely.

In a medium saucepan, combine blackberries, granulated sugar, lemon juice, and cornstarch over medium heat. Stir until thickened and bubbling, about 5-7 minutes. Cool slightly.

Beat cream cheese until fluffy, then gradually add powdered sugar and mix well. Fold in Cool Whip until smooth.

Spread cream cheese mixture evenly over the cooled crust, then top with blackberry filling and sprinkle reserved pretzel crumbs.

Cover with plastic wrap and refrigerate for at least 4 hours, preferably overnight. Slice and serve cold.