Into a bowl mix the 3 eggs with the sugar until light and fluffy. Then add melted butter, milk, and vanilla. Stir in the flour and baking powder and mix together to form a batter.
Put the mixture into greased baking 8 inch tin. Place the diced rhubarb on top of the batter and sprinkle over the top 2 tbsp of sugar.
Bake for 30 minutes at gas mark 4 or 180C. Check the cake has been cooked through to the centre by placing a knife in the centre – if it comes out clean it is ready. If it requires more time keep checking using the knife every 5 minutes until it is ready.