Sift the flour into a large mixing bowl then add sugar and salt.
Heat the milk over the stove until it begins to steam, then add the butter and remove from heat. Stir to help the butter melt.
With the mixer on low, beat the eggs into the flour mixture. Slowly add the warmed milk. The batter will be smooth and runny, runnier than pancake batter.
Cover and refrigerate overnight or at least 3 hours.
Bring batter to room temperature and divide into 5 bowls. Add a few drops of food coloring to each bowl.
Prepare a baking sheet by lining it with parchment paper, and keep your plastic wrap handy.
Heat a non-stick skillet over medium-low.
Drop about 3 tbs batter onto the warmed skillet, then swirl it around to thin it and form a circle. Cook until the bottom of the crepe is lightly browned, about 1 minute. Gently flip then cook the other side for just a few seconds. Remove to the baking sheet to let cool.
Once the baking sheet is full, transfer the cooked crepes to a plate. Layer plastic wrap between each crepe to prevent sticking.
Vanilla Cream Filling
Combine the egg, flour, sugar, and cornstarch in a small bowl, then beat until smooth.
In a small saucepan, heat the milk over low and bring to a simmer. Remove from heat, then quickly whisk in the egg mixture. While continuing to whisk, return the saucepan to the heat and cook for 5 minutes, stirring constantly. Let it thicken, but if it comes to a boil, remove it from heat.
Stir in the vanilla extract and hot water, then set aside to cool until firm.
Once firm, whip the heavy cream, then fold into the custard. Refrigerate until very thick, generally overnight.
Add cherries, water, and sugar to a medium saucepan, then bring to a boil over medium heat. Stir occasionally.
Meanwhile, whisk together the lemon juice and cornstarch in a small bowl. Whisk this into the cherries, then return the cherries to a boil. Stir frequently to prevent it from sticking to the bottom.
Cook for another minute to let thicken, then remove from heat. Let cool completely before using as a topping.
Rainbow Mille Crepe Cake Assembly
Place blue crepe on a cake plate. Using a spatula or butter knife, spread on a thin layer of the vanilla cream. Repeat, until all the crepes and cream has been used.
Refrigerate the cake for 2 hours to let everything set, this will help with slicing it later on.
When ready to serve, top with the cherry sauce and slice.