Preheat oven to 350°F.
Combine brown sugar, water, butter and salt in a medium saucepan. Bring to a boil over medium heat; reduce heat and simmer 5 minutes. Remove heat and stir in vanilla. Set aside.
Combine flour, sugar, baking powder, cinnamon and salt in a large mixing bowl. Mix well. Combine milk, melted butter and vanilla in a small bowl. Add liquid ingredients to the flour mixture; blend quickly.
Spread cake batter in a 13×9-inch cake pan that has been sprayed with cooking spray. Sprinkle with rhubarb. Pour sauce over top but do NOT mix into the batter. Sprinkle pecans over the liquid.
Bake 20 to 25 minutes or until the center bubble up. Serve warm. Top with ice cream or whipped cream, if desired.