Preheat oven to 350 degrees F and grease one 9-in square pan.
Dice your rhubarb and set aside.
In a mixing bowl, cream together butter and sugar until fluffy.
Beat in egg.
In a separate mixing bowl, whisk together flour, baking powder, baking soda, and salt.
Add half of the flour to the creamed mixture and mix. It doesn't have to be fully incorporated. Then add half of the buttermilk and mix. Continue alternating until flour mixture and buttermilk are done. Don't overmix.
Fold in chopped rhubarb and spread evenly in prepared pan.
For the streusel, in a small bowl, mix together flour, sugar, and melted butter until crumbs form.
Sprinkle evenly over the cake and bake for 30-40 min or until golden brown and a toothpick inserted in the center comes out clean. Cool.
While the cake is baking, make the caramel sauce. In a saucepan, over medium heat, melt butter. Add sugar and evaporated milk. Bring to a gentle boil and cook for 2-3 min, or until thickened. The caramel will thicken more as it cools. Once done cooking, stir in vanilla.
Slice the cake and pour caramel sauce over each individual slice. Enjoy!