First, cut the butter into small pieces & leave to soften at room temperature for 30 minutes - 1 hour.
Next, place the butter, tahini, both the sugars & the vanilla into the bowl of a stand mixer. With the paddle attachment, mix the butter & sugar on a medium speed until fully combined, no more than 2 minutes. It’s important not to incorporate too much air at this stage as it will affect the texture of the baked cookie.
With the mixer still running, add in the egg & yolks & mix to combine.
Add in the flour, baking powder, bicarbonate of soda & salt and mix on a low speed until just combined.
Roughly chop 100g of the dark chocolate & all of the milk chocolate then use a spatula to stir through the cookie dough. Cut the remaining dark chocolate into 15 even chunks then set aside.
Combine the granulated sugar & sesame seeds in a bowl. Set aside.
Using a cookie scoop, portion the dough into 70g pieces. You should get 15.With your hands, roll a portion of cookie dough into a ball then press a dark chocolate chunk into the top.Repeat with the remaining dough. Next, roll each ball of cookie dough in the sesame seed sugar, making sure that they are completely coated then place into a container & refrigerate overnight
The next day, preheat an oven to 180°c/160°c fan.Place 3-4 balls of cookie dough onto a lined baking tray, making sure that there’s plenty of room between them. Bake the cookies for 9 minutes then open your oven door, lift up the baking tray inside the oven by a few inches then let it drop down against the oven rack. Use enough force so that the cookie starts to deflate. (This is the “Pan Banging” method.)Bake for a further 2 minutes then repeat the pan banging process.Bake for another 2 minutes (bringing the total bake time to 13 minutes) then remove from the oven. Using a large, round cookie cutter, scoot each cookie into a perfect round shape (this step is optional but makes the cookies more uniform).Sprinkle each cookie lightly with Maldon salt, leave to cool slightly then transfer to a wire rack to cool completely.Repeat the cooking process with the remaining cookies, baking 3-4 cookies at a time.