Place the cut rhubarb pieces into the caramel sauce in the baking pan. Set side.
In a bowl mix the dry ingredients with the lemon zest. Set aside.
In another large bowl beat the butter until fluffy and light. Add the eggs and beat well.
Add half of the flour mixture and beat for one minute.
Add half the milk and beat again.
Add the last half of the flour and beat mixture once more.
Add the final milk and beat until well incorporated.
Carefully spoon the cake batter over the rhubarb and caramel in the baking pan.
Sprinkle with the crumb topping.
Bake 350 degrees for about 40 minutes or until a toothpick inserted comes out clean.
Let the cake cool in the pan for 10 minutes. Then loosen the edges with a knife and invert onto a cool rack.
Let cool completely. Then enjoy!