For the crepe batter:In a large bowl, whisk flour and salt. Whisk in vanilla, eggs and egg yolks until smooth. Gradually pour in milk while whisking, and whisk until well mixed. Whisk in melted butter.Cover and refrigerate for at least 30 minutes, and up to 48 hours.To fry the crepes:Preheat oven to 200ºF (95ºC). Line an oven-proof dish with a large piece of parchment paper. Remove batter from fridge and give it a gentle whisk.Heat an 8-inch cast iron skillet over medium-low heat. Add a small amount of butter to the pan, let it melt, and swirl it to cover the bottom of the pan. Pour a small amount of the batter into the middle of the pan—a small ladle works really well for this job. Immediately tilt the pan to swirl the batter so it evenly covers the bottom of the pan. Cook until the crepe begins to get some color, about 1–3 minutes. Using an offset spatula, loosen the edges of the crepe, lift it, and then use your hands to flip it (be careful not to burn yourself!). Cook until the second side begins to get some color as well, about a minute.Transfer the crepe to the prepared dish, cover with parchment paper, and put in the oven to keep warm.Repeat with remaining crepe batter.Note: If you don't plan to serve the crepes immediately, don't keep them warm in the oven. Instead just keep them in a stack on a plate next to the stove and store in a resealable baggy in the refrigerator once they cool. Reheat in an oven-proof dish, wrapped in parchment paper, in a 200ºF oven for about 20 minutes.