Prepare the crepe batter: In a medium bowl, whisk together the rye flour and salt, then form a well in the dry ingredients. Add the eggs to the well, then begin whisking, gradually bringing in a bit of the flour mixture and drizzling in the milk as you go. Once all the milk has been added, whisk in ½ cup cool water followed by the melted butter. Cover and refrigerate for at least 30 minutes or up to 24 hours. (The batter should be slightly thicker than heavy cream; if it seems too thin, whisk in a tablespoon or two more rye flour.)
Cook the crepes: Heat an iron crepe pan or a non-stick skillet until over medium-high heat. Grease the pan very lightly with oil or clarified butter, then pour in about ¼ cup of the batter. Immediately swirl to coat the surface, then let the crepe cook until the edges start to brown and release from the sides (1–2 minutes). Flip and continue cooking 30 seconds more. Transfer the crêpe to a plate and set aside as you repeat with the rest of the batter, stacking the pancakes neatly as you go. Once all the batter has been cooked, tent loosely with plastic wrap and set aside while you prepare the plums.
Cook the plums: Pit the plums and cut them into bite-sized pieces; cut larger plums into wedges and smaller fruit into halves or quarters.
In a large skillet over medium heat, combine the sugar and ¼ cup water. Bring to a boil over medium heat, and let cook, swirling the pan from time to time so the syrup colors evenly. Cook the caramel until it becomes an amber color, about 8–12 minutes, then add the plums in an even layer. Cook the plums without stirring until the plums release their juices and dissolve the caramel into a glossy sauce, 3–4 minutes. Stir the plums gently, then continue cooking until they have softened and the skins start to separate from the fruit, 8–10 minutes more. Add the lemon juice and cook for another minute. Turn off the heat, tthen pour the brandy into a glass or measuring cup before adding the to the skillet (do not pour directly from the bottle). Using a long lighter, ignite the plums and let them continue to cook in the residual heat, shaking the pan occasionally, until the flames die down, 2–3 minutes. 5. To serve, push the fruit to one side of the pan. Dunk each crepe in the sauce, fold in quarters, and transfer to a platter. Add a spoonful of plums to the top of each crepe and serve warm.