01 -
Switch your oven on to 325°F (165°C). Line up a 9x9-inch pan with a parchment sheet and set it aside for now.
02 -
Pop the butter and chopped chocolate bar into a microwave-safe bowl. Heat in short 20-second bursts, stirring after each, until it turns smooth and completely melts.
03 -
Grab another bowl and whip together sugar, eggs, and the extra yolk. Keep going hard until it turns slightly pale (about 30 seconds). Add in the vanilla, then mix in the melted chocolate mixture until it’s all one smooth batter.
04 -
In a separate bowl, mix up the flour, salt, and baking powder with a whisk. Slowly stir this into the wet batter. Lastly, fold in the chocolate chips using a spatula till combined.
05 -
Spread the batter in the pan you prepared earlier. Place in the center of your oven and bake at 325°F for 42 minutes. Check with a toothpick; it should either come out clean or have a few moist crumbs sticking. Let it cool before cutting.