Preheat your oven to 350°F (175°C). Grease a 9-inch square baking dish with butter or non-stick spray.

Spread chopped fresh rhubarb evenly across the bottom of the greased baking dish. Sprinkle granulated sugar over the rhubarb and drizzle with lemon juice.

In a mixing bowl, combine all-purpose flour, baking powder, and salt. Whisk until evenly combined.

In a separate bowl, whisk together milk, melted butter, and vanilla extract until smooth. Fold in the dry mixture until just combined. Pour evenly over the rhubarb layer.

Mix brown sugar and cornstarch together and sprinkle this mixture over the batter.

Pour boiling water over the entire surface of the baking dish without stirring.

Bake for 40-45 minutes or until the top is golden brown and bubbly. A toothpick inserted should come out clean.

Remove from the oven and let cool for 10 minutes before serving warm with ice cream or whipped cream.