Preheat oven to 320 degrees F (160 degrees C).
Fill a saucepan with a few inches of water, and bring to a simmer.
Place bowl on top of saucepan. Then add Unsweetened Cocoa Powder (65 gram), Unsalted Butter (150 gram), Granulated Sugar (250 gram), and Salt (1/4 teaspoon). Allow the butter to melt completely.
Let rest until the butter has melted and stir.
Remove bowl from heat.
Stir in Vanilla Extract (1 teaspoon).
Add Large Egg (2), one at a time. Stir well to combine.
Add All-Purpose Flour (70 gram).
Add Chocolate (75 gram) if using. Stir thoroughly to combine, about one minute.
Pour mixture into floured baking tin.
Spread out mixture evenly in the pan.
Place in the oven and bake for 40 minutes, or until just set.