Dice pineapple into fine pieces. Place it in a large strainer over a large bowl for 10 to 15 mins. Let sit until most of the residual liquid is released. Drain and discard the liquid, and transfer the pineapple into a large saute pan. Add sugar and salt. Cook over medium heat. The pineapple will release a lot of liquid at first. Continue to simmer, stirring occasionally to avoid sticking, until all liquid evaporates completely and the pineapple pieces start to feel sticky. Add butter and stir to melt. Transfer everything to a medium-sized bowl. Let cool slightly. Then cover the top of the pineapple with plastic wrap. Let cool in the fridge completely before using.