Place the softened butter, caster sugar and orange zest into a large mixing bowl and beat with a wooden spoon until pale and creamy (2-3 minutes). (Electric handheld beaters can also be used).
Add the eggs, one at a time, beating between each addition until completely combined and fluffy.
In a small bowl, whisk together the hazelnut meal, flour and baking powder until combined.
Fold in half of the hazelnut mixture to the butter mixture, until just combined.
Add the orange 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 tightly with foil and bake for 1 hour and 30 minutes, or until the cake is completely cooked through and springs back when gently pressed in the middle. 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.
Remove the cake from the oven and carefully run a knife along the edge of the tin to gently loosen the cake. Carefully turn the cake upside down onto a wire rack (orange slices facing up).
Place a tray underneath the wire rack, to catch any excess orange syrup.
Using a wooden skewer, poke holes into the top of the cake, about halfway deep. Slowly pour the orange syrup over the top of the warm cake and allow it to soak in to the holes.