Preheat oven to 350° Fahrenheit. Place butter in 9-in square baking dish and place in oven to melt.
Combine (frozen) blackberries, ¼ cup sugar, cornstarch and set aside.
Whisk together flour, 1 cup sugar, milk, and vanilla. Set aside.
Remove melted butter from oven and spoon flour mixture into pan. Spoon blackberries and syrup on top of flour mixture.
Bake for 45 minutes - 1 hour or until golden brown on top.
Allow the cobbler to cool for approximately 10 minutes before serving.
Video
Notes
If using fresh blackberries, combine them with ¼ cup sugar and cornstarch and let them sit for 15 minutes before using. This will help them to release their juices in a process called maceration. If you like lemons, feel free to add a little lemon zest to the batter for a light, refreshing taste.