1. Make the filling: Preheat oven to 375 degrees. In a large bowl, combine filling ingredients and toss to coat. Transfer to a baking dish or dutch oven and set aside.
2. Make the topping: In a medium bowl, whisk together flour, cornmeal, sugar, baking powder, baking soda, and salt. Add butter, using your fingers in a snapping motion to rub butter into dry ingredients. Work quickly so the butter doesn’t melt. Add buttermilk to dough mixture and use a fork to stir until evenly combined and dough begins to come together. Using an ice cream scoop or large spoon, scoop dough onto berries, leaving a bit of room between each round.
3. In a small bowl, stir egg until whites and yolk are evenly combined. Gently brush egg wash over dough, then sprinkle with demrara sugar. Bake until filling is bubbly and biscuits are deeply golden brown, about 45 minutes. Allow to cool slightly; serve warm, topped with vanilla ice cream.