Preheat your oven to 375°F. NOTE: you will turn the oven down to 350°F just before baking. Line 2 large baking sheets with parchment paper.
Combine chocolate and butter in a heat-safe bowl and melt in the microwave in short bursts with frequent stirring until completely melted, smooth and glossy. Set aside, but don’t worry about cooling it down - it should still be slightly warm.
Place eggs and sugar in a medium bowl and beat or whisk for 2-3 minutes until pale and thickened and glossy. You should feel that the sugar is becoming dissolved if you rub it between your fingers - this will make fudgy cookies. If you prefer chewier cookies, best for 4-5 minutes until very pale and thick and fluffy (nearly tripled in volume) using an electric hand mixer or using the whisk attachment of a stand mixer. Add slightly warm chocolate mixture and fold it in until evenly blended.
Sift together corn starch, cocoa, baking powder and salt in a small bowl and blend well. Stir it into the chocolate mixture. Fold in chocolate chips. At this stage, the batter will be quite loose like thick brownie batter and not like a traditional cookie dough. That’s is what you should expect. Cover the bowl and set aside for 10 minutes. If you live in a hot environment, then refrigerate for just 5 minutes until the batter is thick and fudge-like but not firm. It will still be fairly loose, but it should roughly hold its shape when you scoop it out (see video for reference).
Tip: If you chill too long then the chocolate will harden and the batter will be too firm and crumbly to scoop (but it will still work - see my tips in the blog post and watch the full recipe video to see both ways). The short chilling time is enough to prevent the cookies from spreading out too much as they bake.
Scoop up about 1 ½ tablespoons of the fudgy batter onto prepared baking sheets. Turn the oven down to 350°F and bake for 8-11 minutes until shiny and crackled on top and set around the edges. Transfer baking sheet to a cooling rack and let cookies cool completely on trays before transferring to the rack individually since the cookies are delicate while warm.