Start by browning the butter. To do this, chop the butter into evenly sized pieces then place into a medium saucepan. Place over a medium heat & allow to melt, stirring regularly with a spatula. Keep cooking, until the butter starts to foam, smell nutty & you can see that the milk solids in the bottom of the pan have turned golden brown.Remove from the heat then stir in the milk powder & miso. Transfer to a bowl & leave to cool to room temperature. Next, mash the banana up with a fork then place into the bowl of a stand mixer along with the cooled butter, both the sugars & the vanilla. Using the paddle attachment, beat together until combined & smooth.Try not to incorporate too much air into the mix at this stage. A maximum of 2 minutes of mixing on a medium speed will do the trick. With the mixer still running on medium, add in the egg & yolk then mix to combine.
Add in the flour, cinnamon, baking powder, bicarbonate of soda, espresso powder & salt then mix on a low speed until just combined.
Roughly chop 100g of the dark chocolate & all of the milk chocolate then add into the cookie dough & stir through. Refrigerate the dough for 30 minutes to firm up.
In the meantime, roughly chop the remaining dark chocolate into 15 chunks. Place the granulated sugar & sesame seeds into a bowl & stir to combine.
Once the cookie dough has been chilled, use a cookie scoop to portion out into 15, 75g balls, making sure that there is plenty of chocolate in each portion.
Press a dark chocolate chunk into the top of each cookie then roll in the sesame sugar. Place onto a baking tray & 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 7 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 4 minutes then repeat the pan banging process.Bake for another 3 minutes (bringing the total bake time to 14 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.