Preheat the oven to 190°C, fan 170°C, gas 5. Grease and line a loose-based, or springform, 20cm tin with baking paper. Whisk the oil and caster sugar in a large bowl, then whisk in the egg yolks and milk, and fold in the pineapple, macadamia nuts and white chocolate. Combine the polenta and ground almonds in a medium bowl, then sift in the baking powder. Stir into the pineapple mixture.
Whisk the egg whites in another bowl until they are stiff, and fold them into the cake mixture. Transfer the mixture to the cake tin and bake for 30-35 minutes until shrinking from the sides and a skewer inserted into the centre comes out clean. Run a knife around the top edge of the cake, and leave it to cool in the tin.
To make the topping whip cream to form stiff peaks, then blend in the Greek yoghurt.
Place the brown sugar into a pan with the 8-10 whole macadamia nuts, until the sugar melts and caramelises the nuts. Remove from the heat and cool.
Just before serving, spoon the topping over the cake. Scatter over the caramelised nuts and dried rose petals.