Preheat over to 350°F and prepare your 8x8 inch pan.
In a saucepan, add butter and rhubarb and simmer over low heat until butter is melted.
Add in chocolate chips then immediately remove from heat. Stir until all chocolate chips are melted.
In a separate bowl, mix eggs, sugar, and vanilla. Add a spoonful of the rhubarb mixture to the egg mixture to temper it. (you don't want your egg to scramble)
Now add your tempered egg mixture to the saucepan of rhubarb and mix well.
Add in cocoa, flour and salt and mix until just combined.
Bake for 25-35 minutes in a prepared 8x8 inch pan, or until a toothpick comes out with minimal crumb. ( I baked mine in a convection oven for 31 minutes.)