Melt the butter and set aside. Using a blender, mix the eggs, milk and flour at low speed. Add the orange liquor. Once the batter is well mixed and coats the spoon with a thin film, refrigerate, covered for 1 hour in a pitcher or bowl.
Stir the batter thoroughly and before making the crepes, test the liquid by dunking a spoon in the mixture; again, it should coat the spoon with a thin film. Adjust by adding milk.
Using at least two and up to four 7 to 8-inch nonstick pans, spray with oil, and heat the pans over medium-high heat. Pour about one cup of batter to coat one pan. Once covered, pour the batter out and into the next heated pan, pouring the remainder back into the pitcher or bowl. The thin layer of batter that remains on the pans will make a smooth, flexible crepe. When the crepes pull away from the edge of the pan (10 to 30 seconds), remove from the heat, tap them lightly or run a knife across the edge of the pan to release, and invert the crepes on a dry towel to cool. Stir the batter at brief intervals to re-suspend the ingredients.