First, make the caramel. Pour sugar and water into a medium pot, but do not stir. Place pot on stove over medium heat, cooking until mixture turns medium brown. NOTE: The caramel can go from perfect to burnt rather quickly, so keep a close eye on it.
Pour the caramel into 6 small ramekins. (We used 3.5" x 1.5" ramekins.) If needed, gently rotate each ramekin to spread the caramel across the bottom. Set on wire rack to allow caramel to harden while you make the custard.
Heat oven to 350°F.
In a medium saucepan over medium heat, heat up the milk until it almost boils, stirring occasionally. Remove from heat.
In a large bowl, whisk together the eggs and the sugar. Slowly pour a small amount of hot milk into egg yolk mixture while whisking constantly. (This gently raises the temperature of the eggs so that they don’t end up scrambled.) After egg yolk mixture has been tempered, pour in remaining milk while continuously whisking.
Strain the mixture to remove any wayward lumps, then mix in piña colada flavour.
Pour custard over caramel, evenly distributing amongst the ramekins. (There will be approximately 100g of custard mixture in each ramekin.)
Place ramekins into a large baking pan (such as a 9" x 13" dish). Gently pour hot water around the ramekins, being extremely careful to not get water into the ramekins. The water should reach about halfway up the side of the ramekins.
Bake for roughly 50 minutes. The purin is done when it’s mostly set but the centre is still a little wobbly.
Taking care to not get any water into the ramekins, remove them from baking dish and place on a wire rack. Allow to cool to room temperature. Then cover with cling film and transfer to refrigerator to chill for at least 3 hours (or preferably overnight).
To serve, slide a knife around the edges of the pudding to loosen. Place a small dish upside-down on top of the ramekin, then—while securely holding both the ramekin and dish—quickly flip. Gently lift up the ramekin, and your pudding will be sitting on the plate in a pool of caramel. NOTE: If your pudding is being shy, fill a pan with hot water and dip the bottoms of the ramekins into the hot water for 5–10 seconds to loosen the caramel.
Serve with whipped cream and a cherry on top, if you like!