Preheat oven to 180°C or 160°C fan. Grease and line a deep 20cm square pan.
Using an electric beater, cream butter and beat in sugar and vanilla extract until pale and creamy. Add eggs, one at a time, beating well after each addition. Fold in flour and a pinch of salt.
Spoon mixture into prepared pan. Bake for 50 minutes, until cake shrinks slightly from sides of pan. Cool in pan for 5 minutes, then turn out onto a wire rack to cool completely.
Make passionfruit icing by combining icing sugar and passionfruit pulp in a small heatproof bowl. Mix well until thick and smooth. Beat in butter and place over a pan of simmering water, stirring, until glossy and smooth. Pour over cake and leave to set before cutting.