Preheat your oven to 350°F (175°C) and prepare a 9-inch deep-dish pie dish with baking spray.

In a large bowl, combine fresh rhubarb, sugar, cornstarch, and lemon juice. Mix well and transfer to the prepared pie dish.

In another bowl, whisk together flour, baking powder, and salt until well combined.

Cream together softened butter and sugar with a hand mixer until light and fluffy. Add the egg and vanilla, mixing until incorporated.

Gradually add dry ingredients to the creamed mixture, mixing gently until just combined.

Spoon the topping over the rhubarb filling, distributing it evenly.

Bake for 35-40 minutes, until the topping is golden brown and the filling is bubbly.

Allow the cobbler to cool for about 20 minutes before serving.