Place the butter, and sugar and orange zest into a large mixing bowl and beat with a wooden spoon until pale and creamy.
Add the eggs, one at a time, beating between each addition until completely combined and fluffy.
Combine the hazelnut meal, flour and baking powder into a bowl and stir to combine.
Fold in half of the hazelnut mixture to the butter mixture, until just combined.
Add the juice and sour cream and stir until combined.
Fold in the remaining hazelnut mixture until just combined.
Pour the cake mixture into the cake tin, over the top of the sliced oranges.
Cover the cake tin firmly with foil and slow bake for 1 hour and 30 - 1 hour and 40 minutes, or until the cake is completely cooked through. You can test by inserting a wooden skewer into the centre of the cake. If it comes out clean, with a few crumbs, the cake is ready.
When the cake is almost done, return the orange syrup to a low simmer for around 5 minutes, until slightly thickened. Add the syrup to a pouring jug.
Remove the cake from the oven and carefully turn it onto a wire rack. Using a wooden skewer, poke several holes in the cake, about halfway down. Using a tray directly beneath the cake/wire rack to catch any stray drips, slowly pour the hot orange syrup over the top of the cake and allow it to soak in.