Preheat oven to 375°. Use about 4 Tbsp of butter to heavily coat the bottom and halfway up the sides of a 10 inch cast iron skillet. Set aside.
Arrange the blackberries in the bottom of the skillet and sprinkle with lemon juice.
Use a fork to stir together egg, sugar, and flour in a medium bowl until the mixture resembles wet sand or a coarse meal. Sprinkle this "crumbly crust" evenly over the berries.
Mix cinnamon and brown sugar and sprinkle evenly over the top of the crumble. Drizzle melted butter evenly over the top of the cobbler and bake for 35 minutes. Let cool for 10 minutes and serve with whipped cream or vanilla ice cream.