For the Dutch Baby: Combine eggs, milk, flour & salt in a blender and blend until completely smooth. Add butter into a preheated cast iron pan, pour batter in and bake for 15mins.
For the Mac Sauce Hollandaise: In a bowl, add egg yolks, vinegar, mustard and salt. Whisk and add butter, ketchup, sweet cornichons and set aside.
Garnish: In a medium heat pan, sauté ground beef. Slice sweet onions, sweet cornichons, lettuce, cheddar and set aside. Once the Dutch Baby is ready, top with Mac Sauce hollandaise and the rest of the garnish.