Preheat the oven. Preheat the oven to 375°F (190°C) and position the rack in the center.
Make the filling. Combine the blackberries, sugar, and cornstarch in a large mixing bowl. Then, toss gently until the berries are evenly coated.
Prebake the filling. Next, pour the filling into the 10-inch cast iron skillet. Cover the skillet with foil and bake the filling for 20 minutes. This step helps release the juices and thickens the filling.
Make the topping. While the filling is pre-baking, prepare the cookie dough topping. In a medium bowl, beat the softened butter, sugar, and vanilla extract with an electric mixer for 2-3 minutes until the mixture is light and fluffy. Add the egg and mix until just incorporated. Gradually add the flour, baking powder, and a pinch of salt, and mix at low speed until combined. The topping should resemble a moist cookie dough.
Assemble and bake. Carefully remove the skillet from the oven (remember, it’s hot!). Carefully spoon out a little filling, let it cool, and taste. You may need to add more sugar depending on the sweetness of the berries. Next, drop spoonfuls of the cookie dough topping over the hot filling. Return the skillet to the oven, uncovered, and bake for 40-50 minutes, until the topping is golden brown and set.
Serve. Allow the cobbler to cool for at least 20 minutes before serving. This dessert is delicious and served warm or cold, ideally topped with a scoop of creamy vanilla ice cream.