Sift the polenta, flour, baking powder, and salt into a bowl. Mix the yoghurt with the oil, lemon juice and lemon zest in a container. Mix the eggs with the egg white and 200 g sugar until creamy, stir in the yogurt mixture and then fold in the polenta flour mixture. Pour the dough into a loaf pan and bake for 40-45 minutes.
Bring the rest of the sugar to the boil in a saucepan with 200 ml of water and the sprigs of rosemary and simmer for 10 minutes. Allow to cool completely and then pour through a sieve into a bowl.
Let the cake cool for 15 minutes, then poke holes in the cake with a chopstick and drizzle half of the rosemary syrup over the cake.
Add the remaining syrup to the cake and drizzle over the individual slices.
You can also e.g., Serve with yoghurt and fresh raspberries.