Start by slicing off the pineapple’s crown and base to stabilize it. Use a sharp knife to carefully remove the thick, fibrous skin, and trim out the “eyes” from the flesh. Chop the pineapple into sections, including the core, and process them in a food processor until the pieces resemble a coarse puree.
Transfer the pineapple puree into a saucepan and cook over high heat to evaporate the liquid quickly. Stir occasionally to prevent burning. Once most of the juice has reduced, lower the heat to a gentle simmer and continue stirring as the paste thickens.
Incorporate the sugar, lemon juice, and maltose into the mixture, blending well to create a rich sweetness with a hint of tang. Add the unsalted butter for a glossy, smooth consistency. Cook until the paste thickens and reaches a jam-like texture.
Remove the filling from the heat and allow it to cool to room temperature. Refrigerate for at least an hour, or until firm enough to handle easily.