Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

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

Add the eggs one by one, mixing well after each addition, then stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt, then gradually incorporate into the wet mixture.

Toss the chopped rhubarb in 2 tablespoons of flour, then gently fold it into the batter.

Spread the batter into the prepared pan and bake for 45-50 minutes until a toothpick comes out clean.

Let the cake cool in the pan for 1-2 hours, then dust with powdered sugar before serving.