Preheat oven to 375 degrees F. Spray an 8x8-inch baking pan with baking spray.
Blueberry Filling Mixture
In a medium sized bowl, add the blueberries, sugar, flour and cinnamon together and toss together coat blueberries.
Cobbler Topping
In a large mixing bowl, using a hand mixer or stand mixer, start by beating softened butter, brown sugar, and granulated sugar in a stand or hand mixer until it becomes creamy (about 2 minutes).
Mix in the egg and vanilla and beat until combined.
Add the flour, baking powder, and salt and continue mixing until combined. Add boiling water and mix together to combine.
Then, add spoonfuls of thick batter, smoothing it out and let some of those delicious blueberries shine through. Finally, top it off with a sprinkle of granulated sugar for that perfect balance of sweetness. Bake for 40-45 minutes until the top is golden brown and the filling is bubbling.