Preheat the oven to 350 degrees with the rack in the middle and line a baking sheet with a silicone mat or parchment paper.
In a medium-size bowl, beat together the butter, tahini, and brown sugar with an electric mixer on medium speed. Beat until thoroughly blended and lightly fluffy, about 1 minute.
Next, add the egg and vanilla and beat just until combined.
Sprinkle in the flour, baking powder, baking soda, and salt evenly over the the dough.
Beat or mix with a spoon until just combined.
Finally, stir in the chocolate chips. Save a few to add on top of the cookies for aesthetic purposes.
Press the dough flat in the bowl and roughly divide it in quarters by eye (you can also lightly score it with a knife). Make three dough balls out of each quarter of dough (for a total of 12 cookies).
Roll the dough balls lightly in your hands and then place them on the baking sheet, evenly spaced. I usually do two rounds of 6 cookies. Cool the cookie sheet between batches. Top with more chocolate chips.
Bake for about 11 minutes. The edges will start to brown, but the centers will still appear soft. Let the cookies cool on the baking sheet for 1 minute before moving them to a wire rack to cool completely.