Grease a 7.5-inch pudding mould with butter.
In pan heat water and brown on low heat with constant stirring. Cook for 2-3 minutes until brown sugar melts completely into a syrup.
Let the syrup cool to room temperature then add coconut milk, mix well and set aside.
In a bowl beat 5 eggs with a whisk and add the brown sugar syrup. Mix well.
Now place the strainer over the greased mould and strain the pudding mixture simultaneously filling the mould.
Cover the mould with a foil paper.
Fill a large pot ¼ with water and bring water to boil. Place the pudding mould in the pot and place a heavy lid on the pot so steam cannot escape. Cook for 40-50 minutes until pudding set completely but still jiggly upon shaking.
Remove mould from steam bath and bring it to room temperature. Then place it in the fridge to chill for 4-5 hours.
When Watalapam is chilled, run a knife across the edges to loosen the mould.
Place a plate on the mould then flip the mould to remove the pudding.
Spread whipped cream on pudding, garnish with pistachios and serve chilled.