Grease & line an 8" square baking tin with baking parchment then set to one side.
Roughly chop up the chocolate & butter then add into a large heatproof bowl along with the tahini & salt. Set the bowl over a pan of barely simmering water then cook until just melted, stirring frequently.Make sure not to let the bottom of the bowl touch the water! Once the chocolate is melted, take the bowl off the heat then whisk in the cocoa powder & espresso powder. Set this to one side & leave to cool slightly whilst you whisk the eggs.
Next, add the eggs, both of the sugars & the vanilla into the bowl of a stand mixer then whisk on a medium speed until the mix reaches the ribbon stage. The eggs will be thick & pale & when you lift the whisk out it will leave a visible ribbon on top. This will take 4-5 minutes of whisking.If you haven't got a stand mixer, an electric hand mixer will work as well. Next, turn the mixer speed down to medium-low then slowly pour in the melted chocolate & mix until just combined. Once all of the chocolate has been added, give the bowl a quick scrape down with a spatula then whisk again briefly.
Take the bowl off the mixer, sift in the plain flour then gently fold in, until just combined. Add in the chocolate chips then fold again, to incorporate.
Tip the batter into the lined tin then give it a firm tap on your counter to pop any large air bubbles. Let the brownie batter rest for 10-15 minutes, whilst your oven preheats.If your oven takes longer than 15 minutes to preheat, turn it on earlier! Next, preheat an oven to 180°c/160°c fan (356°f/320°f). Then once up to temperature, bake the brownies for 20-25 minutes, until the edges are set & the middle still has a slight wobble. It's best to check the brownies after 20 minutes.
Once cooked, set the tin onto a wire cooling rack, leave to cool to temperature then chill in the fridge overnight (or for at least a couple of hours).
When you're ready to portion the brownies, lift them out of the tin & remove the baking paper. Use a large chef's knife to cut into pieces then finish with a sprinkle of sea salt. For clean cuts, dip your knife in hot water & dry between each cut.