Preheat your oven to 375F. Place the blackberries in a 9-inch round baking dish or an 8x8-inch dish. Toss them with the granulated erythritol, lemon juice, and vanilla extract. Spread evenly in the dish.
In a large bowl, whisk together the almond flour, coconut flour, granulated erythritol, baking powder, and salt. Add the cold cubed butter and cut it in using a pastry cutter or your fingers until the mixture resembles coarse pebbles.
In a small bowl, whisk together the egg, heavy cream, and vanilla extract. Pour into the flour mixture and stir until a thick, slightly sticky biscuit dough forms.
Drop heaping spoonfuls of the biscuit dough over the blackberries, spacing them out and leaving small gaps between each mound so steam can escape and the filling bubbles through. Sprinkle the tops with a light dusting of ground cinnamon.
Bake for 28 to 32 minutes until the biscuit topping is golden brown and the berry filling is bubbling up around the edges.
Let the cobbler cool for 15 minutes before serving. Serve warm with a dollop of whipped cream or a scoop of keto vanilla ice cream.
