Preheat oven to 350 degrees. Spray a 9 x 13-inch cake pan with cooking spray and set aside.
Prepare topping:
Combine sugar, cinnamon, and nuts in a small bowl. Set aside.
Prepare cake:
Cream butter and sugar together using a mixer.
Add the egg and blend well.
Combine baking soda and buttermilk in a small bowl. Then add milk mixture to butter mixture, along with salt. Mix well.
Add flour to milk/butter mixture in two batches, mixing until well blended. (It won't be perfectly smooth.)
Fold rhubarb into batter with a spatula.
Spread batter in prepared pan, then sprinkle topping evenly over cake.
Bake cake 35 to 45 minutes, or until a knife inserted in the middle comes out clean.
Let cake cool, then serve topped with whipped cream (if desired). Enjoy!
Notes
If you are like me, you probably don't use a lot of buttermilk. Here's an easy substitute: combine 1 tablespoon lemon juice or white vinegar with enough milk to make 1 cup. Let sit for 5 minutes before using in recipe.