Blend the sugar, salt, eggs, milk, melted butter and vanilla in a blender for 3 seconds. Add the flour and blend until combined, 5 – 10 seconds more. If any flour remains on the sides, scrape down and blend again for a few seconds until combined.
Pour batter into a covered container and refrigerate one hour or overnight.
When ready to cook the flap jacks, preheat a 10″ non-stick skillet or crepe pan on medium low heat until the pan reaches 325º or until a drop of water “dances” across the surface. If it evaporates immediately, the pan is too hot. Turn down the heat and try again. Brush the surface of the pan with melted butter and pour 1/2 cup of batter in pan. Swirl pan to cover the bottom evenly with batter. Cook until the edges begin to brown. Flip over the flap jack and cook until the other side browns lightly.