Place the entire, unopened can of condensed milk in a large saucepan. Fill the saucepan with water and bring to a boil. Lower heat to a simmer and cook until milk inside has caramelized, about 2 hours.

Remove from the heat and set aside to cool while you make the cookies.

Preheat the oven to 325°F (165°C).

Beat butter and sugar together in a bowl with an electric mixer until creamy. Mix in egg, then mix in flour until well combined.

Drop batter by small teaspoonfuls onto an ungreased cookie sheet. Dip your finger into flour, and pat each cookie into a very thin, little circle (about the size of a quarter).

Bake in the preheated oven until just starting to brown around the edges and no longer doughy in the centers, 10 to 12 minutes. Cool on the cookie sheet for 1 minute before removing to a wire rack to cool, 8 to 10 minutes more.