Preheat oven to 150°C (300°F). Line a 30 x 40 cm jelly roll pan with parchment paper.

Melt butter and let it cool slightly.

In a large bowl, whip sugar and eggs together until light and fluffy.

Sift in vanilla sugar and cocoa powder. Add cognac and stir to combine.

Sift in flour and stir to combine. Gradually add the melted butter and stir until well combined.

Pour the batter into the prepared pan and spread evenly.

Bake in the preheated oven for about 35 minutes, or until a toothpick inserted into the center comes out with moist crumbs.

Let the brownie bars cool completely in the pan before cutting into bars.

The bars can be frozen for up to 3 months.