Heat the olive oil in a medium saucepan over medium heat. Add the onion and cook for 5–6 minutes, stirring occasionally, until soft and translucent without developing significant colour.
Add the garlic and cook for approximately 1 minute, until fragrant.
Add the diced pineapple and cook for another 3–4 minutes, stirring occasionally, until the pineapple begins to soften and release some of its juices.
Stir in the brown sugar and ketchup and cook for approximately 1 minute. Add the apple cider vinegar, paprika and cayenne pepper and bring the mixture to a brief boil.
Reduce the heat to a gentle simmer and cook until the glaze has visibly reduced and begun to thicken. When you drag a spoon across the bottom of the saucepan, it should leave a trail for a moment before the glaze flows back together. Don't worry about bringing it to a finished BBQ-sauce consistency at this stage—the glaze will continue to reduce and concentrate as the pork belly finishes cooking.
Remove from the heat and stir in the unsalted butter until completely incorporated. Season with kosher salt, tasting and adjusting if necessary.
The finished glaze should be tangy and slightly sweet, with enough acidity to cut through the richness of the pork belly.