Line a 12-section muffin tray with paper muffin cases. Put the butter, caster sugar, eggs, flour, baking powder and vanilla extract in a bowl and beat with a hand-held electric whisk for about a minute until light and creamy. Divide the mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20–25 minutes or until risen and just firm to the touch. Transfer to a wire rack to cool.
Halve 2 of the passionfruit and scoop the pulp into a bowl with the cream and 1 tablespoon of the icing sugar. Whip until the cream only just holds its shape.
Peel away the cases from the cakes and split each cake in half horizontally. Sandwich together with the passionfruit cream.
Scoop the pulp of the remaining 2 passionfruit into a bowl. Gradually beat in the remaining icing sugar until you have a thin icing and spread over the cakes.