Preheat your Blackstone Griddle and Pellet Grill Combo to 350°F.
Add 1 stick of butter to a large baking dish and place it in the pellet grill to melt.
On the griddletop over medium-high heat, add a large pan, the remaining stick of butter, brown sugar, rum, lemon juice, and lemon zest. Cook for 5-6 minutes until the sugar dissolves.
Add all of the berries to the pan and cook for 10 minutes.
In a large mixing bowl, whisk together the flour, granulated sugar, vanilla extract, and milk until smooth.
Remove the baking dish from the pellet grill. Pour the flour mixture into the baking dish, then pour the berries and their liquid over the top.
Cook the cobbler in the pellet grill at 350°F for 45-50 minutes.
