Combine in a saucepan, 2 cups fresh blackberries, 1/2 cup sugar, 1 tablespoon lemon juice, and 1 tablespoon cornstarch. Cook over medium heat until the mixture starts to bubble and thicken, about 5-7 minutes.

Remove from heat and set aside to cool slightly.

In a large mixing bowl, whisk together 2 cups all-purpose flour, 1 tablespoon baking powder, and 1/2 teaspoon salt.

Cut in 1/4 cup cold and cubed butter until the mixture resembles coarse crumbs.

Stir in 3/4 cup milk and 1 teaspoon vanilla extract until just combined.

Preheat the oven to 350°F (175°C).

Spread the blackberry filling evenly in a baking dish.

Drop spoonfuls of the dumpling dough over the blackberry filling, covering it as much as possible.

Bake in the preheated oven for 25-30 minutes, or until the dumplings are golden brown on top.

Serve warm, optionally with whipped cream or ice cream.