Preheat your oven to 350°F (175°C). Grease and lightly flour a 9x9-inch square baking pan or line it with parchment paper for easy removal.

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

In a large mixing bowl, beat the softened butter and sugar together until light and fluffy (about 2–3 minutes).

Beat in the egg and vanilla extract until fully incorporated. Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients, mixing just until combined. Be careful not to overmix.

Gently fold in the diced rhubarb until evenly distributed throughout the batter.

Pour the batter into the prepared baking pan and smooth the top with a spatula. Bake for 40–45 minutes , or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Let the cake cool in the pan for 10 minutes before transferring it to a wire rack.

Make the Butter Sauce

In a small saucepan over medium heat, melt the butter. Stir in the brown sugar and cook for 2–3 minutes, stirring constantly, until the sugar dissolves and the mixture is smooth.

Gradually whisk in the heavy cream and continue cooking for 1–2 minutes, until the sauce thickens slightly. Remove from heat and stir in the vanilla extract.

Slice the cake and drizzle generously with the warm butter sauce. Serve immediately for the best experience.