I’m not usually finicky about the steps because, trust me, this cake will taste great no matter what. However, you do have to be careful about one thing: getting it out of the pan. Why? Because there’s whipped eggs and sugar, which, when paired with heat, can make a meringue-like glue that will hold onto your cake pan for dear life. It’s essential to use parchment paper to line your cake pan and to grease the absolute heck out of it too. Also, use either an 8″ spring form cake pan or a baking tray (one which you’ll later cut and serve the cake from).
Preheat the oven to 350°F. Trace out the bottom of your 8" springform pan or tray pan onto parchment paper. Ensure when cut and placed to see if it fits, that it's trimmed enough so it lays flat.
Remove parchment, grease the pan, then lay the parchment back into the pan and grease it too (Just grease the whole damn thing, up, down and all around).
Whip the eggs and sugar, lemon zest and chopped rosemary until uber airy. Like, triple the size it once was (don't worry about over mixing, not possible!)
Then slowly stream in lemon juice, olive oil and vanilla extract while beating.
Add the ground hazelnut, flour, baking powder and baking soda. If using a gluten free flour, don't worry about over mixing. If you use AP flour, stir until ingredients are just incorporated (due to gluten in AP flour, overmixing can happen here).
Pour into prepared pan and bake for 45 minutes, or until the top no longer jiggles.
Remove from oven and let cool in pan. Just before serving, sprinkle the top of the cake with powdered sugar (shaken through sieve).