Preheat the oven to 350 and prepare a casserole dish with cooking spray. Set to the side.
Make the peach filling. Combine the peaches with the cornstarch, lemon juice and zest, cinnamon, and salt in a saucepan. Cook the peach mixture on medium-low heat for 5 minutes, until the peaches are just soft.
Make the biscuit topping. Combine the flour, sugar, baking powder, baking soda, rosemary, and salt in a large bowl. Add in the cold cubbed butter. Mix the butter with the flour using a pastry cutter or your fingers. You want the butter to mix into the flour mixture to make pea-sized crumbs. Pour the buttermilk overtop the mixture. Mix to combine gently.
Pour the peach mixture into the prepared casserole dish. Then, using a spoon, add dollops of the biscuit mixture on top of the peaches. Continue adding the biscuit mixture until the peaches are fully covered. Brush with the egg wash and place the cobbler in the oven.
Bake the peach cobbler in the oven for 40-50 minutes, until golden brown. Serve warm with vanilla ice cream.
