1. Preheat oven to 350°F (175°C). Grease a 9×9-inch baking pan and line with parchment if desired.

2. Trim and wash rhubarb, then chop into ½-inch pieces to measure 2 cups. If using frozen rhubarb, thaw and drain off excess liquid.

3. In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

4. In a large bowl, beat softened butter and sugar together until light and fluffy, 2–3 minutes.

5. Add eggs one at a time, beating well after each, then mix in vanilla.

6. With the mixer on low, add half of the flour mixture. Add buttermilk, then the remaining flour mixture, mixing just until combined.

7. Gently fold rhubarb into the batter. Spread batter into the prepared pan and smooth the top.

8. Bake 35–45 minutes, until golden and a toothpick inserted in the center comes out with a few moist crumbs.

9. For the sauce, melt butter in a small saucepan over medium heat, then stir in sugar and cream.

10. Bring to a gentle boil and cook 3–4 minutes, stirring often, until slightly thickened, then remove from heat and stir in vanilla and a pinch of salt.

11. Let the cake cool at least 15–20 minutes before cutting. Serve warm slices with warm vanilla butter sauce spooned over the top.