Preheat your oven to 350°F (175°C). Gather all ingredients and prepare a greased 9x13-inch pan.

In a large mixing bowl, cream softened unsalted butter and granulated sugar using an electric mixer until light and fluffy.

Add eggs one at a time, mixing well after each addition, then blend in vanilla extract.

In a separate bowl, whisk together all-purpose flour, baking powder, baking soda, and salt.

Gradually add the dry mixture to the creamed mixture, alternating with buttermilk, mixing gently until just combined.

Fold in chopped rhubarb gently until evenly distributed.

Pour the batter into the greased pan and bake for 40-45 minutes, or until a toothpick comes out clean.

Let the cake cool in the pan for 30 minutes before transferring to a wire rack to cool completely.