Heat oven to 375°F. Place butter in a 9x13-inch baking dish and melt in the oven.
Toss blackberries with 3/4 cup sugar and the lemon juice. Set aside to macerate.
Whisk flour, remaining 1/2 cup sugar, baking powder, and salt in a bowl.
Stir in milk and vanilla until a smooth, pourable batter forms.
Remove hot pan and swirl to coat with butter. Pour in the batter; do not stir.
Spoon berries and their juices evenly over the batter. Dust the top with cinnamon.
Bake 40–45 minutes until the top is deep golden and the juices bubble. Cool 10 minutes before serving.