Preheat your oven to 350°F (175°C).

Grease and flour a 9x13 inch baking pan.

In a mixing bowl, cream together 1/2 cup softened butter and 1 cup sugar until light and fluffy.

Beat in the eggs, one at a time, letting each one fully disappear before adding the next.

Stir in 1 teaspoon vanilla extract.

In a separate bowl, whisk together 2 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt.

Add the dry mix to the creamed mixture alternately with 1 cup buttermilk: a little dry, a splash of buttermilk, repeat. Stir just until combined.

Gently fold in 2 cups chopped rhubarb.

Pour the batter into the prepared pan and smooth the top.

Bake for 35–40 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

In a small saucepan, melt 1/2 cup butter over medium heat.

Add 1 cup sugar and 1/2 cup heavy cream; stir constantly until the sugar dissolves.

Remove from heat and stir in vanilla if using.

Let the cake cool slightly, then cut into squares and drizzle generously with warm butter sauce.