In a large bowl, combine the blackberries with 1/2 cup of sugar, lemon juice, and vanilla extract. If you prefer a thicker filling, add cornstarch. Toss to coat the blackberries evenly and set aside to macerate for 10-15 minutes.
In another large bowl, whisk together the flour, remaining 1/2 cup of sugar, baking powder, salt, and cinnamon until well combined.
Pour in the melted butter and milk, stirring until just combined. Do not overmix; the batter should be slightly lumpy.
In a large, oven-safe skillet, pour the batter evenly over the bottom, then spoon the blackberry mixture over the batter. Do not stir; the blackberries will sink into the batter as it bakes.
Bake in the preheated oven for 35-40 minutes, or until the cobbler is golden brown and a toothpick inserted in the center comes out clean.
Remove from the oven and let it cool for about 10 minutes before serving.
Notes
Serve warm directly from the skillet, topped with a scoop of vanilla ice cream or whipped cream, and garnish with a sprig of mint for a pop of color.