Over a bain-marie (a bowl placed over a pot of gently simmering water), melt 205g of dark chocolate and butter, stirring until well-combined and shiny; set aside
Sift dark brown sugar, corn starch, baking powder, spices and salt into a bowl and whisk gently to incorporate
In a stand mixer or by hand, whisk eggs into the dry ingredients one by one, whisking vigorously between additions; the mixture should feel silky and not gritty when rubbed between your fingertips
Gently pour in the melted butter and chocolate mixture and whisk until homogenous
Roughly chop 70g white chocolate, ginger candy and walnuts and gently fold into the batter; cover mixture with cling-wrap and chill in the fridge overnight
To portion, use a spoon to scoop out 45g of chilled cookie dough, roll it into a ball over the palm of your hand and place onto a tray lined with baking paper (each tray should have around 8 cookie balls)
Using the palm of your hand, gently press down on each cookie ball to flatten the surface slightly, and add a small pinch of salt over the top of each cookie
Place into an oven at 170°C (ideally near the top) for 5 minutes, flip the tray for even heating, then turn up the oven to 180°C to bake for another 5 minutes
The cookie should still look slightly undercooked when taken out of the oven, allow to cool until the top becomes crispy and the cookie no longer sticks to the baking paper; serve warm