01 -
Preheat the oven to 350°F (175°C). Grease a 9-inch square baking pan and line it with parchment if you want cleaner slices.
02 -
In a medium bowl, whisk together the flour, baking powder, and salt until evenly combined.
03 -
In a large bowl, beat the softened butter and sugar for 2-3 minutes until lighter and fluffy. Beat in the egg and vanilla.
04 -
Add half of the flour mixture, then the milk, then the remaining flour mixture. Stir gently after each addition just until no dry streaks remain.
05 -
Fold in the chopped rhubarb. Spread the batter evenly in the prepared pan and smooth the top.
06 -
Bake for 45-50 minutes, until the top is lightly golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.
07 -
While the cake cools slightly, combine butter, sugar, heavy cream, and salt in a small saucepan. Cook over medium heat, stirring often, until smooth and lightly bubbling, 5-7 minutes. Remove from heat and stir in vanilla.
08 -
Cut the warm cake into squares and spoon warm butter sauce over each serving. Add vanilla ice cream or whipped cream if desired.