In a large mixing bowl, cream together the butter, dark brown sugar, and granulated sugar until slightly lightened in color. (No need for it to be super fluffy—just well blended and smooth.)
Add the eggs one at a time, beating well after each addition.
Pour in the crème de cacao and the Guinness reduction syrup, and beat until fully incorporated.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Gradually mix the dry ingredients into the butter mixture until a soft dough forms.
Stir in your desired amount of chopped chocolate (chunks, chips, or a mix—it’s your moment).
Wrap the dough tightly in plastic wrap and refrigerate for at least 2 hours (overnight if you want even deeper flavor).
When ready to bake, preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
Scoop dough by tablespoonfuls, spacing about 1 inch apart on the prepared baking sheet. For extra flair, press a few extra pieces of chocolate onto the tops of each cookie.
Bake for 8–10 minutes, or until the edges are just set but the centers still look slightly soft.
Let cool on the baking sheet for a few minutes, then transfer to a wire rack to cool completely.