Line a rectangular baking sheet of about 15.8 x 11 inches (40 x 28 cm) with parchment paper.
Fill a pot with water, put it on the stove over medium heat and let it boil. Place a heat resistant medium size bowl on top without it touching the water, to have a double boiler, so it melts, stiring every so often. Once it is totally melted, take away from the heat and let it cool down.
In a big bowl, place the granulated sugar, brown sugar, salt, eggs, and vanilla extract and beat using a mixer for about 5 minutes until the mixture thickens and turns lighter in color.
Add the chocolate and stir until combined using a rubber spatula.
Sift together the flour, baking powder and cocoa powder and add them to the mixture. Incorporate using the rubbaer spatula until you get a soft mixture. If you are using chocolate chips, add them here.
Cover the bowl with some plastic wrap and refrigerate for 30 minutes. Preheat the oven to 356º F (180º C).
Once this time has passed, using a spoon, make balls with the mixture (trying not to overheat the mixture with your hands) of about 1.77 inches (4.5 cm) in diameter and place them on the baking sheet with a separation of at least 2 inches (5 cm) between each other. It may be needed to bake the cookies in two batches.
Bake for 10 to 12 minutes, not any longer, otherwise they will turn out stiff. In the middle of baking, take them out of the oven and slam the baking sheet on the kitchen countertop lightly to get a better texture, then put back again in the oven so they fully cook.
Take the cookies out of the oven and place them on a cooling rack so they completely cool down. Here´s where those small cracks will form.