In a large saucepan, combine the blackberries, sugar, lemon juice, and water. Stir gently and bring to a simmer over medium heat.
Optional thickening: If you want the sauce to be thicker, mix 2 tablespoons of cornstarch with 2 tablespoons of cold water to make a slurry. Stir the slurry into the blackberry mixture and cook until the sauce thickens (about 2-3 minutes). If you prefer a thinner sauce, skip this step.
Add the cinnamon if desired, and stir to combine. Reduce heat to low to keep the blackberry mixture warm while you prepare the dumplings.
2. Make the Dumpling Dough:
In a medium bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon (if using).
Add the cold butter cubes to the flour mixture. Use a pastry cutter, forks, or your hands to cut the butter into the flour until the mixture resembles coarse crumbs with pea-sized butter pieces.
Pour in the milk and vanilla extract. Stir gently until just combined, being careful not to overmix. The dough should be thick and a bit sticky.
3. Cook the Dumplings:
Once the blackberry filling is ready, drop spoonfuls of the dumpling dough into the simmering blackberry mixture. The dumplings should be about 1 to 2 tablespoons each, and they will puff up as they cook.
Cover the pan with a lid and cook over low heat for about 20-25 minutes. Do not lift the lid during cooking, as the steam is essential for cooking the dumplings through. The dumplings should be light and fluffy when done.
4. Serve:
Once the dumplings are fully cooked, remove the pan from the heat. Spoon a generous amount of the blackberry sauce and dumplings into serving bowls.
Optional: Serve with a scoop of vanilla ice cream or a dollop of whipped cream for an extra treat
