In a cast iron skillet, combine blackberries, sugar, water, lemon juice, cinnamon, and vanilla extract. Stir and simmer for about 5 minutes until berries soften.

In a mixing bowl, whisk together flour, baking powder, salt, and sugar. Cut in cold butter until resembling coarse crumbs, then stir in buttermilk and vanilla until moistened.

Drop tablespoonfuls of dumpling dough into the simmering blackberry sauce, then cover and simmer on low for 12 to 14 minutes until cooked through.

Remove from heat and let cool slightly. Serve warm in bowls with sauce, optionally topped with ice cream or whipped cream.