Preheat the oven to 350°F. Grease a 9x13-inch baking dish.
In a large bowl whisk together the flour, baking powder, salt, cocoa powder, and granulated sugar.
Stir in the milk, melted butter, and vanilla until just combined.
Fold in the 15 chopped Oreo cookies.
Spread the batter evenly into the prepared baking dish.
In a medium bowl stir together the granulated sugar, brown sugar, cocoa powder, and the remaining 10 chopped Oreo cookies.
Sprinkle the topping evenly over the batter.
Carefully pour the boiling water evenly over the entire casserole. Do not stir.
Bake for 40–45 minutes, until the top is set but the center is still slightly soft and the chocolate sauce is bubbling underneath.
Remove from the oven and immediately sprinkle the remaining 5 chopped Oreo cookies over the top.
Let the cobbler rest for 15–20 minutes before serving.
Serve warm with big scoops of vanilla ice cream.