Melt the butter, dark chocolate, and oil together in a heatproof bowl. Let it cool slightly.
In a separate bowl, beat the eggs with brown sugar, caster sugar, and vanilla until light and fluffy.
Mix the melted chocolate into the egg mixture.
Gently fold in the flour, cocoa powder, optional coffee powder, salt, and chocolate chunks until just combined. Avoid overmixing.
For the cookie layer, combine melted butter with both sugars, then add the egg and vanilla.
Fold in flour, baking soda, corn starch, salt, and chocolate chunks until the dough forms. Chill briefly if too soft.
Line an 8x8 inch (20cm) pan with parchment paper. Spread half the brownie batter evenly.
Drop spoonfuls of cookie dough over the brownie layer, pressing lightly to cover. Repeat until all dough is used.
Preheat the oven to 175°C (350°F) and bake for 40–50 minutes. The top should be golden but slightly soft in the center.
If the cookie layer browns too quickly, loosely cover with foil.
Let brookies cool completely before slicing to maintain texture.