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, smells nutty & you can see that the milk solids in the bottom of the pan have turned golden brown.
Transfer the brown butter to a small mixing bowl & leave to cool slightly (15 minutes or so).
Next, add both sugars & the milk into the brown butter then whisk until well combined.
Add in the egg yolk then whisk again, until combined & smooth.
Add the flour, baking powder, bicarb & salt into the cookie dough & mix with a spatula, until just incorporated.Be careful not to over mix! Next, roughly chop up ¾ of the chocolate then stir into the cookie dough. Cover the mixing bowl with clingfilm then chill in the freezer for 20 minutes.
In the meantime, preheat an oven to 170°c/150°c fan (338°f/302°f) and chop the remaining chocolate into 2 chunks.
Once the cookie dough has chilled, portion it into two even pieces, roll into balls then press a chunk of chocolate into the top of each one.Each ball of cookie dough should weigh around 100 grams. Transfer to a lined baking tray then bake in the preheated oven for 11-13 minutes, until the cookies are golden, the edges are crisp & the centres are just set.
Once cooked, sprinkle the top with sea salt. Leave to cool then serve!