Start by combining the thawed marionberries with their juice, granulated sugar, water, cornstarch, vanilla extract, and lemon zest in a saucepan. Set over medium heat and stir gently until it bubbles and thickens. This should take about 5 to 7 minutes. Let it simmer as you prepare your cobbler topping.
Preheat your oven to 350°F (175°C) while your filling cools. This ensures the dish starts baking immediately once it's assembled.
In a mixing bowl, combine Pamela’s Pancake & Baking Mix, sugar, and salt. Then, add the softened butter and mix until crumbly. Pour in the milk, cinnamon, and lemon juice. Stir until just combined—do not overmix.
Pour the berry filling into an ungreased 9x13-inch baking dish. Pour the topping mixture over the berry filling unevenly—this will create that gorgeous, rustic look. Bake for 30-35 minutes or until the topping is golden brown and cooked through.
Remove the cobbler from the oven and allow it to cool for about 10-15 minutes before serving. This waiting time is crucial; it helps the juices settle.