Preheat your oven to 350°F (175°C).
In a mixing bowl, cream together the butter and sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition.
Stir in the vanilla extract.
In another bowl, whisk together the flour and baking soda.
Add the dry ingredients to the creamed mixture alternately with the buttermilk, beginning and ending with the dry ingredients.
Fold in the chopped rhubarb.
Pour the batter into a greased and floured baking pan.
Bake for 45 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.