Fudgy Condensed Milk Cookies

In a medium saucepan, combine the chocolate, condensed milk, and butter. Cook over medium heat, stirring occasionally, until the chocolate is fully melted.
Turn off the heat and add the flour, baking soda, salt, espresso powder (if using), egg, and vanilla. Stir to combine well.
Add the nuts and cacao nibs, if using. These are nice additions for crunch but not necessary!
Let the dough cool, and then chill it for about 20 minutes in the refrigerator. Once chilled, divide the dough into four equal portions (or three, if you want larger cookies). Roll each portion into a log, about 2" wide, and wrap tightly in plastic wrap.
Chill the logs for at least 2 hours, or you can pop them in the freezer for 20 minutes if you're in a rush. You just want them to be firm and easy to slice.
When you're ready to bake, preheat the oven to 325 degrees F, and line your baking sheets with parchment paper.
Working with one log of dough at a time, slice the logs into thin slices (about 1/4" to 1/2" each)-the thicker the slice, the chewier the cookies will be.
Place the cookies on the prepared baking sheets-you don't have to leave too much space, as they don't spread much.
Bake the cookies for 12 minutes. They'll look underdone, but that's okay, take them out anyway. They'll firm up as they cool. Repeat with the remaining logs (you can also keep the dough frozen for up to a month before using).