Pre-heat your oven to 400° and line an 20 x 20-centimeter square pan with parchment paper and spray with non-stick cooking spray.
In a double boiler (a metal or glass mixing bowl over a saucepan of simmering water) melt the butter and chocolate. Once melted remove the bowl from the saucepan and set aside to let cool slightly.
In a medium mixing bowl, and using a hand mixer beat the sugar, eggs and vanilla until light and airy - about five minutes. Add the cooled butter and chocolate mixture and continue to beat for another minute.
Sift in flour and cocoa powder. Using a rubber spatula fold in the flour and cocoa powder until it is completely combined - do not over mix. Fold in the salt and one cup of the rhubarb. Pour batter into prepared pan and sprinkle the remaining cup of rhubarb on top.
Bake for 40-45 minutes - these brownies are fudgy!
Remove the pan from the oven and let cool in the pan for 20 minutes. Enjoy!