Preheat the oven to 350 F (180 C).
Grease a 9-inch (23 cm) springform pan and line the bottom with parchment paper.

In a bowl, combine the flour, baking powder, and salt. Set aside.
In another bowl, cream the butter with the sugar using an electric mixer.
Add the eggs one at a time and beat until the mixture is smooth.
At low speed, alternate adding the dry ingredients with the pineapple and rum.

Pour the batter into the pan and bake in the oven for about 1 hour or until a toothpick inserted comes out clean.
Let it cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.

Coconut Milk Sauce:
In a saucepan, bring the coconut milk and sugar to a boil, then reduce the heat and simmer for about 10 minutes or until it has reduced to 1 and 1/4 cups (310 ml).
Transfer to a bowl and let it cool to room temperature.
Cover and refrigerate for at least 1 hour.
To serve, drizzle each slice of cake with the coconut milk sauce.