Add the frozen berries, granulated sugar, brown sugar, cornstarch, vanilla, and salt directly to your skillet or baking dish. Toss until evenly coated. Let sit for 10–15 minutes while preparing the topping.
In a separate bowl, whisk together the sourdough discard, buttermilk, sugar, and vanilla until smooth.
Add the flour, baking powder, baking soda, and salt. Stir just until combined.
Pour in the melted butter and gently mix until a soft batter forms. Do not overmix.
Spoon the batter over the berry mixture in rustic dollops. It does not need to fully cover the fruit.
Sprinkle the top with raw sugar if desired.
Bake for 40–50 minutes, or until the topping is golden brown and the filling is bubbling in the center.
Let the cobbler cool for at least 25–30 minutes before serving. The filling will continue to thicken as it cools.
Serve warm with vanilla ice cream if desired.
Notes
-Frozen berries work wonderfully in this recipe and are encouraged for convenience and year-round baking, but you can also use fresh berries (see notes below).-Do not thaw frozen berries before baking, as thawing can release excess liquid and create a thinner filling.-For a thicker, more jammy filling, use 3 tablespoons of cornstarch. If you prefer a slightly softer filling, 2 1/2 tablespoons is ideal, especially if serving the cobbler warm.-Recent sourdough discard works best for balanced flavor and texture. If your discard is very thin or liquidy, you may need to add an extra tablespoon or two of flour to the topping batter.-For best texture, avoid overmixing the cobbler topping. A gently mixed batter will bake up soft and tender.-Allowing the cobbler to cool before serving helps the filling thicken and set properly.-This is delicious served with homemade vanilla ice cream!How to Store and Reheat:Store leftover cobbler covered in the refrigerator for up to 3 days. Reheat individual portions in the microwave or warm the cobbler in a 300°F oven until heated through.To Use Fresh Berries Instead of Frozen:
Use the same amount: 4 cups (600–650 g) fresh mixed berries.
If your berries are on the firmer side and not especially juicy, you can reduce the cornstarch slightly to 2 tablespoons for a softer, more syrupy filling.
Fresh berry cobbler will most likely finish a little sooner. Bake at 375°F for about 35–45 minutes, It's done when the topping is golden brown, the filling is bubbling vigorously in the center (not just around the edges), and the berries are tender.
Don't skip the 25–30 minute cooling time. Fresh berry cobblers can seem a little loose straight from the oven, but the filling thickens considerably as it cools.
Buttermilk Substitutions: To make 1/2 cup of buttermilk, combine 1/2 cup of regular milk with 1 1/2 teaspoons of lemon juice or white vinegar. Stir together and allow to sit for 5-10 minutes until the milk curdles slightly. You can also substitute 1/2 cup of thinned yogurt for the buttermilk.