Grease a 9x13 inch baking dish (or similar 3-quart baking dish) and spread the cut rhubarb evenly across the bottom. This creates the fruit base that will become the pudding layer as it bakes. Set aside while you prepare the batter.

In a medium bowl, whisk together the flour, baking powder, salt, and cinnamon. In another bowl, combine the melted butter, milk, and vanilla. Pour the wet ingredients into the dry ingredients and stir until just combined—the batter should be slightly lumpy. I like to use King Arthur flour because it gives a more tender crumb, and the cinnamon adds a subtle warmth that complements the tartness of the rhubarb beautifully.

Spread the batter from Step 2 evenly over the rhubarb in the prepared dish. In a small bowl, whisk together the 1 cup sugar and cornstarch until well combined, breaking up any lumps. Sprinkle this mixture evenly over the batter surface. The cornstarch will help create a clear, glossy sauce as it bakes and draws moisture from the rhubarb.

Carefully pour the boiling water evenly over the entire surface of the assembled cake. Don't stir—the water will seep down during baking to create the pudding sauce beneath the cake. I like to pour the water slowly around the edges first, then across the middle to ensure even distribution. Place in a preheated 375°F oven and bake for 45 minutes until the cake is golden and a toothpick inserted into the cake portion (not the sauce) comes out clean.

Remove from the oven and let cool for at least 10 minutes before serving. This resting time allows the pudding sauce to set slightly while remaining warm and spoonable. Serve each portion warm with a generous dollop of whipped cream or a scoop of vanilla ice cream to complement the tart rhubarb.