Make the crepes: Place the flour, eggs, egg yolk, milk, sugar, salt, and melted butter in a blender or food processor and blend until smooth. Add the water and blend again until smooth. Alternatively, you can mix the ingredients by hand. Pour the batter into a container, cover, and refrigerate for at least 30 minutes, or up to 12 hours. When ready to make the crêpes, stir the batter with a whisk or fork until well combined and smooth.
Set a crêpe or non-stick pan over medium-high heat. Once it is hot, butter the surface. Tilt the pan, ladle a scant 1/4 cup batter over the lower side, and quickly tilt and turn the pan to spread the batter over the entire surface of the pan.
Cook for 25 to 30 seconds, or until edges are beginning to dry and the bottom is lightly browned. With a spatula, lift up one edge of the crêpe and quickly turn it over, use the spatula or your fingers to flip it, as if it were a tortilla. Cook the second side for 10 to 15 seconds, or until lightly browned. Remove the crêpe from the pan and place on a plate.
Repeat with the rest of the batter, stacking the crêpes on top of one another. After making 2 to 3 crêpes, you may need to butter the pan again.
Make the dulce de leche sauce: Bring the dulce de leche and milk to a gentle simmer in a medium saucepan over medium heat, stirring occasionally. Simmer for 2 to 3 minutes or until dulce de leche has dissolved. Remove from the heat. Stir in the rum, if using to make for grown up crêpes.
To assemble the crêpes: Place one crêpe on a plate and spread a couple tablespoons of the sauce over the surface. Fold the crêpe in half, add a couple more tablespoons of sauce, and spread it around. Fold the crêpe again to make a triangle shape and pour another tablespoon of sauce on top. Repeat with the remaining crêpes. Garnish with toasted pecans and mixed berries, if desired.