Chop the chocolate and set aside.In a medium mixing bowl, add in the butter, both sugars, vanilla, espresso powder, and salt.
Cream the butter and sugar mixture on medium speed for about 3 minutes until it becomes a few shades lighter. Scrape the sides and bottom of the bowl.
Crack in the egg and mix again until smooth and fluffy.
Dump in the rye flour and then the baking soda.
Use the mixer to mix on low until just combined. It's ok if some flour is still visible.
Add in all of the chocolate except the 12 larger chunks that you'll put on top.I like big cookies so I used a quarter cup ice cream scoop but you could do any size you want, you'll simply need to adjust the baking time accordingly.
Line a small tray or baking sheet with parchment paper scoop the cookie dough onto it.
Once all the dough has been scooped, roll each scoop into a smooth ball.
Press one of the chocolate chunks into the top of each ball.
Cover the tray with plastic wrap and put in the fridge to chill for at least 2 hours. They can stay in the fridge overnight but any longer and I would transfer them to a freezer bag and store in the freezer until ready to bake.