Melt butter in a microwavable dish.
Pour 1 cup of sugar and all of the flour into a mixing bowl, whisking in milk. Mix well.
Then, pour in melted butter and whisk until all ingredients are fully combined. Don’t overmix though. Mix just until the ingredients are incorporated.
Butter a baking dish. I use a 9x12 which works great and creates a thinner cake. If you want a thicker cake, feel free to use a smaller baking dish. A smaller baking dish means it will take a bit longer to cook because the cake will be thicker, so add cooking time in 5 minute increments.
Pour the batter into the buttered baking dish. Sprinkle blackberries over the top of the batter; distributing evenly. Sprinkle ¼ cup sugar over the top.
Bake in the oven at 350 degrees for 1 hour, or until golden and bubbly. If you desire, sprinkle an additional teaspoon of sugar over the cobbler 10 minutes before it’s done.
Serve with a heaping spoonful of vanilla ice-cream, whipped cream or both!