Preheat oven to 350°F (180°C) and line baking pans with parchment paper.
Make brown butter by melting butter over medium heat, adding milk powder, and whisking until golden and nutty. Remove from heat and cool slightly, then stir in whiskey.
In a bowl, mix brown butter, sugar, and salt. Add eggs and vanilla, mixing until thick and slightly lighter in color.
Fold in flour and chocolate chunks to form cookie dough. Set aside.
In another bowl, melt butter, chocolate, sugars, and salt in the microwave in intervals, stirring until smooth.
Add coffee and vanilla mixture, then whisk in eggs until thick and pale.
Sift flour and cocoa powder, then fold into the batter until just combined.
Alternate scoops of brownie and cookie dough into prepared pans and gently swirl, or layer brownie first then cookie dough on top.
Optionally sprinkle sea salt flakes and extra chocolate on top.
Bake for 35–40 minutes until a toothpick inserted comes out with a few crumbs.
Cool completely in the pan, then lift out and cut into bars.
Notes
Use high-quality chocolate for best flavor.
Do not overmix batters to keep texture tender.
Let bars cool fully before slicing for clean edges.
Store in an airtight container at room temperature for up to 3 days.