Preheat the oven to 350°F (175°C) and lightly grease your baking pan.
In a small saucepan, melt the butter with milk and set aside to cool slightly.
In a large bowl, beat the eggs and sugar until light and fluffy.
Sift in the flour, baking powder, and salt, and gently fold into the egg mixture.
Slowly pour in the butter-milk mixture and stir until just combined.
Fold in the chopped rhubarb evenly.
Pour the batter into the greased pan and smooth the top.
Bake for 30–35 minutes, or until golden and a toothpick comes out clean.
Slice and Serve.