01 -
Preheat the oven to 175°C (350°F) and grease a 23cm (9-inch) square baking pan.
02 -
In a medium bowl, whisk together the flour, baking powder, and salt until well combined.
03 -
In a large bowl, cream together the sugar and butter until light and fluffy, about 3 minutes. Beat in the egg and vanilla extract until fully incorporated.
04 -
Gradually add the flour mixture to the creamed mixture alternately with the milk, beginning and ending with the flour mixture. Mix just until combined after each addition.
05 -
Fold in the chopped rhubarb gently until evenly distributed throughout the batter.
06 -
Pour the batter into the prepared pan, spreading evenly. Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
07 -
While the cake is baking, combine butter, sugar, and heavy cream in a saucepan over medium heat. Cook, stirring constantly, until the sugar dissolves completely and the sauce becomes smooth, about 5-7 minutes.
08 -
Remove the saucepan from heat and stir in the vanilla extract. Allow to cool slightly before serving.
09 -
Slice the warm cake and serve with the butter sauce drizzled over each portion.