Preheat oven to 160°C, line 8-inch square metal pan

In a small sauce pan over medium-low heat, melt butter. Swirl the pan occasionally and increase the heat to medium. It should become foamy with audible popping noises. Swirl until butter develops a nutty aroma and brown bits start to form at the bottom. Pour into a bowl and immediately add dark chocolate and stir until melted

Combine brown & white sugar, eggs and vanilla extract. Beat mixture with electric mixer on high speed until completely thickened and fluffy in texture, and lightened in colour (~8 minutes)

On low speed gradually pour in the warm brown butter chocolate mixture

Use a rubber spatula to stir in the flour, cocoa powder and salt until just combined. Stir in the bittersweet chocolate chips

Pour into prepared pan and smooth out with spatula. Bake for 45-50 minutes or until set but not overbaked

Cool in pan before slicing and serving

With non convectional oven, bake at 160° with pan of water at the bottom for 35 minutes, take out pan for final 10-15 minutes.