Combine flour, soda and salt in a bowl.
In another bowl cream together butter sugar eggs and vanilla.
Add buttermilk and flour mix slightly Fold on rhubarb.
Pour onto a sprayed 9x13 pan.
Crumble together butter brown sugar and flour.
Sprinkle on top of batter.
Bake 350 degrees 45 minutes.
Test with toothpick as rhubarb moisture content varies.
Cool 1 hour.
Enjoy as is or with whip topping or ice cream.