Preheat your oven to 350°F (175°C). Chop rhubarb and place in a greased baking dish. Sprinkle with sugar and cinnamon, then toss to coat.

In a bowl, whisk flour, baking powder, and salt. In another bowl, mix buttermilk, melted butter, and sugar until smooth. Fold wet ingredients into dry to create a thick batter.

Pour the cake batter over the rhubarb mixture evenly. Ensure the rhubarb is not visible to promote even baking.

Bring water to a boil and pour it over the cake batter carefully without stirring, to form the pudding layer.

Bake in the preheated oven for about 45 minutes or until the top is golden and a toothpick comes out clean.

Let the cake cool for 10-15 minutes before serving. Serve warm with vanilla ice cream or whipped cream.