1.Make passionfruit marshmallow. Grease two 12-hole (1½-tablespoon/30ml) shallow round-based patty pans. Stir passionfruit, sugar and the hot water in small saucepan, over low heat, until sugar dissolves. Combine gelatine and the cold water in small jug. Pour gelatine mixture into hot sugar syrup; cook, stirring, until gelatine dissolves. Pour mixture into small heatproof bowl; cool to room temperature. Beat mixture and a little food colouring with electric mixer, on high speed, about 8 minutes or until mixture is thick and holds its shape. Working quickly, spoon the mixture into pan holes; smooth surfaces. Stand at room temperature about 1 hour or until firm. Slide marshmallows out of pan holes.
2.Meanwhile, beat butter, sugar, egg and extract in small bowl with electric mixer until combined. Stir in sifted flour and custard powder. Roll dough between sheets of baking paper until 5mm (¼-inch) thick; place on tray. Refrigerate 30 minutes.
3.Preheat oven to 180°C/350°F. Line oven trays with baking paper.
4.Cut 24 x 5.5cm (2½-inch) rounds from dough. Place rounds, about 2.5cm (1 inch) apart, on trays.
5.Bake biscuits about 12 minutes. Cool on trays.
6.Working quickly, spread ½ teaspoon of the melted chocolate onto centre of each biscuit; immediately top with marshmallow, flat-side down. Place biscuits on wire rack over baking-paper-lined tray; pipe or drizzle with remaining chocolate. Stand at room temperature until set.