Place a medium skillet or saucepan over medium heat and add the butter. Let it melt slowly, swirling the pan once or twice. As soon as it starts to foam and you catch that first nutty, caramel-like aroma, add the brown sugar and stir to combine. You want the mixture to look glossy and unified. Don't rush this step; low and steady heat prevents the sugar from scorching.

Tip your pineapple chunks into the pan and stir to coat them in the butter-sugar mixture. Cook for about three to four minutes, stirring occasionally, until the pineapple pieces begin to soften and turn golden at the edges.

Carefully add the dark rum to the pan and give everything a good stir. If cooking on a gas stove, keep your face back as the alcohol can briefly flame up. Let it simmer for two minutes so the alcohol cooks off slightly while the rich molasses notes of the rum meld with the sweet pineapple.

Stir in the vanilla extract, cinnamon, and a small pinch of salt. If you prefer a thicker consistency, whisk together the cornstarch and water in a small bowl, then stir it into the sauce. Cook for one more minute until the sauce coats the back of a spoon. Remove from heat and serve immediately, or let it cool before storing.