1. Preheat your oven to 350°F (175°C).

2. In a large bowl, cream together the butter and granulated sugar until light and fluffy.

3. Add the eggs one at a time, beating well after each addition.

4. Stir in the vanilla extract and milk.

5. In another bowl, whisk together the flour, baking powder, baking soda, and salt.

6. Gradually add the dry ingredients to the wet mixture, mixing until just combined.

7. Fold in the diced rhubarb gently.

8. Pour the batter into a greased 9×13 inch baking dish.

9. In a separate bowl, mix the brown sugar and cinnamon together, then sprinkle it over the batter.

10. Bake in the preheated oven for 35-40 minutes, or until a toothpick comes out clean.

11. Let cool before serving.