Preheat the oven to 160°C. Line a 12-hole muffin pan with large paper cases.,Using a stand mixer fitted with the paddle attachment, beat butter and sugar until thick and pale. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla, then add the sour cream and flour in 3 alternating batches. Fold through the passionfruit pulp.,Spoon batter into prepared pan and bake for 22 minutes or until muffin tops spring back when lightly pressed. Cool to room temperature.,For the icing, place icing sugar and passionfruit in a bowl. Stir until smooth. Spoon over muffins and serve.