Preheat the oven to 350°F with the rack in the center.
In a 9 x5-inch loaf pan or a 10-inch pie plate, combine the berries, ½ cup of the brown sugar, the cold butter cubes, and vanilla. Toss well to mix.
In a small bowl, stir together the melted butter, flour, baking powder, the remaining 1½ tablespoons brown sugar, and the flaky sea salt. Don't worry if the mixture seems dry. Crumble the topping over the berries.
Bake for 45 minutes, or until the berries are bubbling and the topping is golden brown.
Serve warm, with vanilla ice cream.
