Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish.

Combine flour, sugar, and baking powder in a medium bowl; stir in milk until blended. Stir in rhubarb until evenly incorporated. Pour batter into the prepared baking dish; spread into an even layer.

Combine brown sugar, boiling water, and butter in a separate bowl until well combined and butter is melted; pour over batter.

Bake in the preheated oven until a toothpick inserted into center comes out clean, 45 to 50 minutes.