Preheat the oven to 175°C conventional or 160°C fan. Grease and line a 23 × 33cm baking pan.
Melt the chocolate and butter together until smooth. Leave to cool for 5–10 minutes.
Whisk the eggs and granulated sugar together in a large bowl.
Gradually whisk in the cooled chocolate mixture and vanilla.
Fold in the flour just until combined.
Spread the brownie batter evenly in the prepared pan.
For the topping, melt the butter and brown sugar over low heat. Remove from the heat and cool for approximately 10 minutes.
Whisk in the flour and vanilla. Gradually whisk in the beaten eggs.
Fold in the chopped walnuts.
Spread the walnut mixture evenly over the brownie batter.
Bake for 40–45 minutes, until a toothpick comes out with a few moist crumbs.
Cool completely before lifting from the pan and cutting into 18 brownies.