Preheat the oven to 350°F (177°C). Line a baking sheet with parchment paper or a silicone baking mat and set aside.

In a medium bowl, mix together the flour, cocoa powder, and baking soda until evenly combined. Set aside.

Melt the butter in a small pan over medium heat until about halfway melted. Remove from heat and stir until fully melted. Let cool slightly.

Place the sugar in a large bowl and add the melted butter. Mix well until glossy and fully combined.

Add the egg and vanilla extract and stir until smooth and fully incorporated.

Add the flour mixture in two batches, stirring after each addition until the dough comes together evenly.

Roll the dough into 52–53 gram balls (about 2½ tablespoons each) and place on the prepared baking sheet. Alternatively, divide the dough evenly into 10 portions and roll into balls.

Bake for 8–11 minutes, until the cookies are slightly set. The tops should look dry but still feel soft.

Remove from the oven and let the cookies cool on the baking sheet for about 10 minutes, until the bottoms are firm. The cookies will be slightly puffy. If desired, gently press the tops with the back of a spoon while warm.