Preheat oven to 180C/350F. Line four oven trays with baking paper.
Beat butter and caster sugar in a small bowl with an electric mixer for about 5 minutes or until thick and creamy. Beat in eggs, one at a time.
Sift dry ingredients into a large bowl; stir in butter mixture, milk and essence until combined. Drop level tablespoons of the mixture onto the trays, about 5 cm (2 inches) apart.
Bake two trays at a time for 8 minutes or until puffed and golden. Stand on the trays for 5 minutes, then transfer to a wire rack to cool.
Meanwhile make passionfruit spread. Whisk passionfruit, egg yolks, sugar and corn flour in a small saucepan over low heat. Add butter, one piece at a time, whisking until combined. Bring to the boil, then remove from heat. Refrigerate mixture for 30 minutes, stirring occasionally.
Sandwich cakes with passionfruit spread, dust with sifted icing sugar.