Preheat the oven to 350F.
Put a skillet on medium-low heat, melt the butter until it is golden yellow. Add in the miso and stir with a spatula until it is fully combined and no clumps remain.
Continue to cook down the butter and miso until the mixture starts to fizz slightly. Stir constantly as the miso and butter begin to brown. When they get to a golden brown color, remove from the heat and keep stirring until you get a rich dark brown color. Pour this into a large bowl.
Whisk in the milk to the butter, it should turn into a creamy yellow liquid. Allow this mixture to cool slightly before adding in the eggs. Beat well.
Then add in the sugars and vanilla, mix until smooth.
If the bowl is large enough, add in the flour, cornstarch, baking powder and whisk to combine. The dough should be sticky and a little wet feeling, but it will thicken up.
Chop the chocolate and add to the bowl. Mix well.
Scoop the dough into 40 gram sized balls, refrigerate for 10 minutes or until the oven is preheated.
Bake at 350F for 9-11 minutes. You want the cookies to still feel gooey in the center, because the residual heat from the pan will continue to cook them after they are out of the oven.
Serve whenever!