Whisk the dry ingredients. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
Mix the wet ingredients. In a large bowl, use a rubber spatula to mix together the coconut oil, tahini, water, and vanilla until combined, about 1 minute.
Add the sugar, dry ingredients, and chocolate. Mix in the sugar, then gradually mix in the dry ingredients until just combined. Add the chocolate all at once, and mix until the chocolate is evenly distributed throughout, about 1 minute.
Chill the dough while prepping the oven and sheet pans. Cover the bowl with plastic wrap and refrigerate for 10 minutes. Meanwhile, position a rack in the center of the oven and preheat the oven to 350°F. Line two half sheet pans with parchment paper.
Assemble the cookies. Use a 3-Tablespoon cookie dough scoop to portion the cookie dough into balls and place them at least 3 inches apart on the prepared sheet pans. Bake the cookies. Bake one pan at a time for 12 minutes, until the edges have set but the centers are still gooey. The cookies will look puffed when you pull them out of the oven, but will fall and crack into the perfect cookies as they cool. Cool the cookies on the pan on a wire rack for 20 minutes, or until the edges and bottoms of the cookies feel firm to the touch. Repeat with remaining cookie dough balls.
Serve and store. Serve warm or at room temperature. The cookies can be stored, in an airtight container at room temperature, for up to 3 days.