Preheat oven: 350°F (175°C) and line cookie sheets with parchment paper.
Brown the butter: In a saucepan over medium-low heat, melt butter. Continue to cook, swirling occasionally until it turns golden brown with a nutty aroma. Remove and let cool for 5 minutes.
Mix sugars and butter: Combine brown sugar, granulated sugar, and browned butter in a large bowl until smooth.
Add eggs & vanilla: Stir in egg, yolks, and vanilla until creamy.
Incorporate dry ingredients: Whisk flour, baking powder, and salt in a separate bowl. Gradually mix into wet ingredients until just combined.
Fold in chocolate: Gently fold in chocolate chips and chopped dark chocolate.
Chill dough: Cover and refrigerate 30 mins.
Form dough balls: Scoop and roll dough into balls; place on prepared sheets.
Bake: 10–12 minutes until edges are golden but centers are slightly soft. Cool on sheet 5 minutes, then transfer to wire rack. Sprinkle with flakey salt if desired.
Make ghost marshmallows: Melt white chocolate and dip marshmallows. Let harden, then use black icing to draw faces. Once cookies are cooled, dip marshmallow bottoms in chocolate and attach to cookie for a “melted ghost” effect.