This cake has a nice lemony taste and really shows off the tangy passionfruit butter in between its two halves. You can use this spread between cake halves to make a plain cake very special. It's kind of like lemon butter - or lemon curd - but with the tangy taste of passionfruit.
To make the Passionfruit Butter: Place all ingredients in the top half of a double boiler or in metal bowl over a pot of simmering water. Cook for about 15 minutes stirring continuously until it thickens.
To make the cakes: Preheat oven to 190 degrees C. Grease two 20cm cake tins and line with baking paper.
Cream the butter and sugar. Add the eggs one at a time, beating well after each addition. Fold in the flour and the lemon rind.
Divide the mixture between the tins and cook for 25-30 minutes until golden and a skewer inserted in the middle comes out clean.
Cool the cakes for a few minutes before completely cooling them on wire racks.
To assemble: Spread the mascarpone on the top of one of the cakes and spread the passionfruit butter on top of the mascarpone. Top with the second cake and dust the top with icing sugar.