In a large skillet, combine the fresh or frozen blackberries, sugar, water, lemon juice, ground cinnamon, and vanilla extract. Stir gently over medium heat until it starts to simmer, about 5 minutes.

In a medium bowl, whisk together the sifted flour, baking powder, additional ground cinnamon, salt, and sugar until well combined and fluffy.

Add the cold, unsalted butter pieces into the dry ingredient mixture. Work the butter into the flour until it resembles coarse crumbs, about 3-5 minutes.

In a small bowl, mix the buttermilk and vanilla extract until combined. Pour into the dry ingredients and stir gently until just combined, being careful not to overmix.

Drop spoonfuls of the dough into the simmering blackberry sauce, spacing them evenly.

Cover the skillet with a lid or aluminum foil, reduce heat to low, and let the dumplings simmer for 12-14 minutes until they are puffed and cooked through.

Remove from heat and let cool slightly before serving. Serve warm with vanilla ice cream or whipped cream.