Was wondering can the buttermilk be replaced with sour cream or yogurt ? I have made this once with buttermilk it was amazing.

I would not hesitate to make that substitution in this Winter Hazelnut and Orange Cake, but buttermilk is generally a bit more liquid-y, so I would thin the yogurt or sour cream a bit with milk or water. A good rule of thumb is to thin the yogurt or sour cream to the consistency of heavy cream.

If you try it, let us know how it turns out, please. ;o)