In a medium bowl, whisk together 4 large eggs and 1/4 teaspoon kosher salt until uniformly yellow.
Add about half of the flour (around 1 cup) to the eggs, whisking gently into the eggs. The batter will be a little lumpy, but that's okay right now. Add about half of the milk and continue whisking. The milk will thin out the batter. Alternate between the flour and milk until you've added it all. Continue whisking until smooth — be careful not to overmix, it should only take a minute or two.
Once the batter is smooth, add 1/4 cup melted unsalted butter and whisk again until the butter is just incorporated into the mixture. Cover the batter and refrigerate for 1 hour.
Once the batter has chilled, heat a medium (at least 8 inches) non-stick skillet pan on medium heat. The surface of the pan needs to be pretty hot to make crepes — test its heat by adding a drop of water to the hot pan. If the water sizzles and bubbles and evaporates, the pan is hot enough. Pour about 1/4 cup batter into the pan and use the handle to twirl the pan so that the batter forms a circle. As you twirl the pan in a circular motion, the batter should spread and makes the crepe. Continue cooking until the crepe has set and the edges begin to crinkle up — a set crepe will usually slide around the pan when it is ready. Slip a spatula under the crepe to flip it over and continue cooking for another 20 seconds, or until the crepe's surface has browned. Repeat the process until the batter is finished, allowing the crepes to cool on a wire rack completely before frosting and building the cake.