Preheat the oven to 350 degrees (F). Grease a 9-inch square baking dish and set aside.

In a large mixing bowl, cream together the softened butter, 1 cup of the sugar, and vanilla extract. Bead in the eggs one at a time.

In a small bowl, combine 1-1/2 cups of the flour with the baking powder and salt.

Alternate adding the milk and the dry ingredients to the butter mixture. Mix until just combined.

Mix the chopped rhubarb with the remaining 1/4 cup sugar, cornstarch, orange juice, and orange zest in a separate bowl until the rhubarb is well coated.

Spread the cake batter evenly over the bottom of the greased baking dish. Layer the coated rhubarb over the batter.

In a new bowl, combine the oats, remaining 1/2 cup flour, brown sugar, and cinnamon. Using a pastry cutter or fork, cut the chilled butter into the mixture until it resembles sand. Sprinkle the crumbly topping evening over the rhubarb layer.

Bake for 45 minutes or until the cake is golden brown and a toothpick inserted into the center comes out clean.

Allow the cake to cool to room temperature. Serve slices with a dollop of whipped cream.

If you can't find fresh rhubarb, you may substitute frozen rhubarb. Be sure to defrost first and drain excess liquid before using. You may need to increase the baking time by 5 to 10 minutes. Use a toothpick inserted into the center to check for doneness before increasing the baking time.