Step 1The day before you intend to bake the canneles, make the batter: Combine the sugar and flour in a large mixing bowl and whisk to mix evenly. Whisk in the condensed milk, eggs, yolks, rum and vanilla.
Step 2Combine the water, butter and dry milk in a saucepan. Bring to a boil over low heat, whisking occasionally. Whisk the hot mixture into the flour and egg mixture until thoroughly combined. Pass the batter through a fine-meshed strainer into a container; let cool slightly, then cover tightly and refrigerate a minimum of 24 hours, a maximum of 5 days.
Step 3When you are ready to bake the canneles, heat the oven to 400 degrees.
Step 4Remove the batter from the refrigerator and whisk to combine, 1 minute. Fill cannele molds 3/4 full. (If using silicone molds, do not grease; tins should be lightly coated with oil.)
Step 5Bake until the canneles are firm and very dark brown, about 1 hour, changing the position of the pans several times -- back to front and top to bottom during baking. Let sit 5 minutes, then invert onto a rack. Gently pull the sides of the silicone mold out to release the canneles onto a rack. Serve warm.