Combine all syrup ingredients in a small saucepan. Bring to a simmer and cook for 5 minutes. Let cool.
For the Cake
Preheat the oven to 325F. Grease a 12 x 9 pan with vegetable oil.
In a large bowl, beat egg yolks with sugar until light yellow and a ribbon forms. Add oil, yogurt, flour, hazelnuts, walnuts, baking powder, cinnamon, and zests. Stir to combine. The mixture will be very coarse and thick.
In a clean dry bowl, beat egg whites until stiff peaks form. Fold egg whites into the batter and pour mixture into pan. Spread evenly.
Bake 30-45 minutes or until an inserted toothpick comes out clean.
Pierce cake all over with the toothpick. Ladle syrup over the cake slowly, giving the syrup time to absorb into the cake before adding more.
Cool the cake completely, cover with plastic wrap and refrigerate overnight.
Cut the cake into circles or diamonds and decorate with a hazelnut.