Add flour, eggs, salt, vanilla sugar (or vanilla extract + sugar) to a large bowl. Using a whisk, beat the batter as you slowly add the milk. Whisk until there are no lumps. The batter should be on the thin side.
Melt butter in a small saucepan.
Heat a crepes pan or an 8-inch non-stick frying pan over medium high heat. Use a pastry brush to spread a bit of melted butter in the pan. Use a ladle (or a 1/4 cup measuring cup) to transfer batter into the middle of the pan and then quickly swirl the pan so a thin layer of batter fills the entire pan. Let the crepe cook for 1-2 minutes on each side.
Transfer finished crepes to a plate to cool. Place a layer of wax or parchment paper in-between each crepe so they don't stick to each other once they've cooled.
Make at least 20 crepes. Cover crepes with plastic wrap and store in the fridge until you're ready to assemble the cake.
Beat whipping cream and vanilla sugar (or vanilla extract + white sugar) in a medium bowl to stiff peaks. Fold in the lemon curd and zest. Cover and store in the fridge until you're ready to assemble the cake.
Slice strawberries (I like to cut some in half and some in slices). Cut hazelnuts in half or chop into small pieces (if using).
Place one crepe on a plate or cake stand. Spread lemon whipped cream on the crepe. Place another crepe on top and spread another layer of lemon whipped cream. Repeat this process until you've used all 20 crepes. Spread the remaining whipped cream on the top of the cake. Add strawberries and hazelnuts. Just before serving, drizzle golden syrup over the strawberries. This cake is best served the day it's made.