Edit: In the video, I whisk the eggs first, but now I start by melting the chocolate; it’s more practical.
Warning! The melted chocolate should not be hot, otherwise you won’t get that shiny, crackly crust on top of your brownie cookies.
Preheat the oven to 180°C (350°F).
Melt the chocolate and butter in a double boiler or over low heat, stir until smooth, and let cool slightly.
In a bowl, whisk the eggs, vanilla, and sugar with an electric mixer or a hand whisk for about 3 minutes, until the mixture becomes lighter and frothy.
Add a small amount of the egg-sugar mixture to the lukewarm chocolate (almost cold but not hardened) and mix well.
Pour the chocolate mixture into the remaining egg mixture and whisk until smooth and homogeneous.
Sift in the flour and gently whisk it in.
Fold in the chocolate chips. Mix well, then let the batter firm up. The dough should not spread when you drop spoonfuls onto the baking sheet.
Butter a baking sheet and line it with parchment paper.
Place 2 teaspoons of dough per cookie, one on top of the other. The dough should not be runny and will spread very little, if at all.
Press chocolate chips or Smarties on top (optional).
Bake for about 10 minutes. As soon as the cookies develop cracks and a shiny crust on top, remove them from the oven.
Lift the parchment paper with the cookies still on it off the baking sheet, and let them cool before removing them.
Warning: they’re highly addictive!