Place tahini, maple syrup, coconut oil, and vanilla into a large bowl and stir well to combine.
Add baking powder, baking soda, salt, and flour, and stir again to fully combine the dough.
Roughly chop your bar(s) of chocolate- the more variety in shape/size, the better! Some larger chunks mixed in with the smaller bits is great here. Add the chocolate to the dough and stir to combine.
Transfer the bowl of cookie dough to the fridge to chill for 30 minutes (or cover and chil for up to 2 days, if you'd like to make the dough in advance).
When dough is chilled, preheat the oven to 350 degrees Fahrenheit and line two sheet pans with parchment paper.
Use a 1/2 cup measuring cup to portion the dough into 6 large balls. Divide any remaining cookie dough evenly among the 6 balls. Arrange 3 dough balls onto each pan, leaving plenty of space between them.
Bake for 18-20 minutes, or until centers are just barely set and edges are browning. (If both sheet pans fit on the same rack in your oven, bake them at the same time. If not, bake them in two separate batches to ensure even baking!).
Once finished baking, sprinkle each cookie with a pinch of flaky sea salt (optional!). Allow cookies to cool for at least 15 minutes before attempting to move from the pan. Since they're so big, they break easily while still warm! Once completely cooled, cookies can be kept covered at room temperature for up to 4 days.