Whisk together egg, warm whole milk, sugar, salt, and vanilla extract and set aside.
For caramel sauce: Cook ½ cup sugar and 2 tablespoons water until golden. Cook a little longer for a bitter caramel.
Pour caramel into the container and coat the whole bottom. Be careful, caramel is hot.
Slowly add flan mixture through a strainer into the caramel container. Scoop out all the bubbles using a spoon or a fine strainer.
Bring steamer to a full boil. Add in the flan. Turn heat to low, the lowest heat level of the stove.
Steam for 60 minutes with lid. The water should not be bubbling.
Remove water from the flan with a paper towel. Gently let it absorb.
Let flan rest for 3-4 hours to firm up, or refrigerate overnight.
Cut around the edges, cover with a plate and flip flan over.
