Strain the juice from the pineapple, and set aside. Add the sugar, corn syrup, and reserved pineapple juice to a medium saucepan over high heat. Bring to a boil.
Reduce heat to medium-low and simmer, stirring occasionally, until the mixture thickens slightly to a loose mixture, 10-15 minutes. The mixture will thicken somewhat more upon cooling. Add the pineapple pieces and cook for 2-3 minutes. *Add the butter if it starts to foam.
Remove from heat and allow to cool slightly. Pour into a glass jar and cover. Store in the refrigerator for up to 2 weeks.
Notes
This will not be thick like say hot fudge but rather a syrupy sauce with lots of pineapple in it. The sauce will just coat the back of a spoon.