In a mixing bowl, cream together the butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
In another bowl, whisk together flour, baking powder, and salt. Gradually add to the creamed mixture, alternating with milk, beginning and ending with the flour mixture.
Fold in the chopped rhubarb.
Pour batter into a greased baking pan and bake for 45 minutes or until a toothpick inserted into the center comes out clean.