Heat oven to 210'c, grease a muffin tray/cake tin and line with greaseproof paper or muffin cases.
In a medium bowl, beat Butter until creamy then beat in Castor sugar until pale and fluffy. Beat in Eggs, one at a time. Finally beat in Baking powder.
Mix in the crushed Walnut halves and fold in the Plain flour, Semi-skimmed milk and the Passion fruit pulp. Spoon mix into muffin cases/cake tin.
For muffins bake for 25 - 30 minutes, for cake tin bake for 45-60, or until golden brown and not gooey in the middle. Allow to cool before removing from the tray.
In an electric mixer, whisk the Double cream and Vanilla extract until stiff peaks form. Using a sharp knife, cut a hole into the top of the muffin, remove some of the cake and cut the top of the removed piece, keeping as a lid (extra cake can be discarded or eaten). Spoon in the cream and close using the top of the muffin that was kept.
Melt the butter and sieve in the Icing sugar and add the Passion fruit pulp and combine all of the ingredients. Stir until all combined, if too thick add a few drops of boiling water. Apply to muffins and keep refrigerated until eaten.