Preheat oven to 180°C. Grease a deep 25cm fluted tube cake pan.

Beat butter, rind and sugar in a small bowl with an electric mixer until pale and fluffy. Beat in eggs, one at a time, until just combined. Fold in curd, sifted flours, salt and milk. Spoon mixture into pan.

Bake cake for 45 minutes or until a skewer inserted into the centre comes out clean. Stand cake in pan for 15 minutes before turning out onto a wire rack to cool.

Meanwhile, to make sesame seed crunch, line an oven tray with baking paper. Stir sugar and the water in a small saucepan over medium heat until sugar dissolves. Bring to the boil. Reduce heat; simmer until syrup turns a golden caramel. Immediately pour mixture onto tray. Sprinkle with sesame seeds. Stand until set. Finely chop a quarter of the crunch; break remainder into small pieces.

To make mascarpone frosting, beat ¾ cup (180ml) cream in a small bowl with an electric mixer until soft peaks form. Combine mascarpone and sifted icing sugar in a medium bowl; fold in whipped cream, then remaining cream.

Level base of the cake if necessary, then place on a platter. Spoon half the frosting on cake; decorate with sesame seed crunch. Serve cake with remaining frosting.