Preheat the oven to 350°F(177C°). Line baking sheet with parchment paper or silicone baking mat. Set aside.
Put the flour, cocoa powder and baking soda in a small bowl and mix well. Set aside.
Melt the butter in a small pan over medium heat until it is halfway melted. Take it from the heat and stir until it is all melted. Set aside to cool.
Place the sugar in a big bowl and add the butter, mix well. Then add in the egg and vanilla, stir until combined.
Add the flour mixture in 2 batches and stir until all combined well.
Roll the dough into 52-53 gram (2 and ½ tablespoon) balls and place them on the baking sheet. Alternatively, you can divide the dough equally into 10 pieces and roll each into a ball.
Bake for 8-11 minutes or until the cookies are slightly set. The top of the cookies shouldn't be wet but soft. They will be too soft to handle. Remove the cookies from the oven so that they can keep their moistness. Let them cool on the baking sheet for 10 minutes or until the bottom of the cookies are firm enough. The cookies will be a little bit puffy. You can slightly press the cookies with the back of a spoon.