Peel and quarter the pineapple and remove the stalk. Cut the pineapple into thin slices. Melt butter in a pan, place pineapple slices in it, sprinkle with 2 tbsp. sugar and let it caramelise for about 2 minutes, turning occasionally.
Stir mascarpone, 3 tablespoons sugar and orange juice until smooth. Pour rum and 3 tablespoons of water into a deep plate. Cut 4 sponge fingers into pieces and dip them briefly in rum mixture. Line dessert glasses with dipped ladyfingers.
Arrange the pineapple on the glasses. Crumble 2 biscuits and distribute into the glasses.
Add mascarpone cream and refrigerate for about 2 hours. Just before serving, dust with cocoa powder and decorate with sugar pearls.