Roughly chop the chocolate and set aside.
Preheat the oven to 350°F / 180°C / Gas Mark 4, no fan (if you have a convection oven reduce the temperature by about 20° or follow the manufacturer's instructions).
Add the softened butter to the mixing bowl and both types of sugar. Mix over medium-low speed for about 3 minutes, until light, fluffy, and well-combined.
Add the egg and egg yolk, mix over medium-low speed for another 2-3 minutes, until well-combined. Scrape the sides of the bowl from time to time with a spatula.
Stir the flour with cinnamon, baking soda, and salt. Add the flour mixture to the mixing bowl. Mix over low speed until half combined (you should still see flour here and there and the dough will be rather clumpy).
Add the bourbon and chopped chocolate to the bowl. Mix until the chocolate is evenly distributed and the flour fully incorporated (but don't mix for too long).
Chill the dough in the fridge for about 40 minutes or longer (you can bake them straight away but they will be quite flat).
Scoop the dough with a 3-tablespoons-ice cream scoop or just with your hands/spoon (optionally, you could weigh your dough, each ball of dough should weigh 47g). Place on a baking sheet lined with parchment paper about 2 inches (5 cm) apart, don’t flatten the cookies. If using chocolate chips, press a couple of chips into the dough balls (this is optional, just to make them more 'pretty').
Bake the cookies in batches for about 13-14 minutes. The baking time may slightly vary depending on the oven. The cookies should have golden edges but will be completely soft. While the cookies are still soft and hot, put 2-3 chunks of chocolate on top of each cookie, and press them slightly in (optional). Sprinkle the cookies with flaky sea salt.
Take the baking tray out of the oven and let the cookies slightly cool on the baking sheet (they will continue to set up from the residual heat on the pan after they come out of the oven). Transfer to a cooling rack to cool completely. Always use a cold baking sheet for the next batch of cookies.
Enjoy!