Before embarking on a chocolate-thumbprint-cookie baking expedition, I had to first make dulce de leche. This is largely a simple process, if one could even call it a process. It is simply just a matter of having enough time and patience to wait.
All you need is a large pot, water and a can of sweetened condensed milk.
Remove the label from the can, put it in a large pot filled with water (it should be about 2 inches or 5 centimeters above the top of the can, so the can is completely submerged). Bring the water to a boil. Once it reaches a boil, lower the temperature on the stove so that the water simmers. Cover the pot with the lid.
Now you just have to wait, letting the can simmer at least two or three hours (I left mine for about four hours on the lowest heat). After you have turned off the heat, leave the pot to sit for about 30 minutes. Then run the whole pot and can under cold water in the sink.
Only once the can is cool enough to handle should you lift it out of the pot. Then just let it stand. To be on the safe side, I left it overnight before opening it. Opening it up, I had perfect, creamy, thick dulce de leche just waiting to fill the chocolate thumbprint cookies.