Preheat the oven to 350 degrees F. Butter an 8x8 brownie pan, bottom and sides.
Pour the melted butter into a large bowl.
Whisk in brown sugar until smooth.
Add the egg and vanilla extract, whisk again until combined.
Add the flour and mix until no lumps are visible.
Throw in the diced rhubarb and chocolate chips and fold through gently with a wooden spoon or spatula.
Transfer the mixture to the prepared pan and spread it out to make even.
Sprinkle some of the sea salt over the surface.
Bake in the oven for 30-35 minutes.
Let cool for at least 20 minutes before slicing.
Dust with powdered sugar or drizzle with extra white chocolate (optional)