Preheat the oven to 350°F (177°C). Line a baking sheet with parchment paper and set aside.
Chop the chocolate bar if you haven't already and set it aside.
Whisk the flour, baking soda, instant coffee granules and salt together in a bowl, and set aside.
In a large bowl, add room temperature butter, brown sugar, and granulated sugar and beat on medium for about 3 minutes. Make sure to scrape the sides of the bowl in between mixing to get all the ingredients incorporated. After 3 minutes of beating, butter and sugar will become fluffy and lighter in color.
Add the egg and vanilla extract, mix until incorporated.
Add in the dry ingredients, mix on low until the flour is just combined. It's ok if you still see traces of flour left, we'll fold those in with chocolate later on.
Fold in the chocolate chunks with a wooden spoon or a spatula.
With a cookie scoop or a spoon, scoop the cookies on the parchment paper, sprinkle some flaky sea salt on top (optional but makes them so much better!). Make sure to leave enough room between the cookies because they spread while baking. Bake in the preheated oven for 6 minutes, take them out add a couple of chocolate chunks on top of each cookie, rotate the sheet and bake 4 more minutes (Total bake time: 10 minutes).
Let the cookies cool down completely and enjoy!