½cupsemisweet or dark chocolate chips(255g) plus additional for topping, if desired
Combine your butter and your unsweetened chocolate in a large microwave-safe bowl and heat for 25 seconds. Stir well and then return to the microwave, heating for 15-second increments and stirring well after each one, until chocolate and butter are completely melted.
Immediately add cocoa powder and instant coffee and stir well. Allow mixture to cool for several minutes before proceeding.
Add your sugars and stir well.
Add egg, egg yolk, and vanilla extract and stir until well-combined. Set aside.
In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet and stir until well-combined. Stir in chocolate chips.
Cover bowl with plastic wrap and transfer to the refrigerator to chill for 30 minutes.
While dough is chilling, preheat oven to 350F (175C) and line a baking sheet with parchment paper.
Once dough has finished chilling, scoop by heaping 1 ½ Tablespoon-sized scoop and roll briefly between your palms to form a smooth round ball.
Place cookie dough at least 2" apart on baking sheet and bake on 350F (175C) for 10-11 minutes. Cookie should still seem very soft in the centers and almost under-baked, they will finish cooking as they cool on the baking sheet so let them cool completely on the baking sheet (they will be too fragile to move before they've cooled).
If desired, gently nestle additional chocolate chips into the top of each cookie within 1-2 minutes of removing cookies from oven.
Keep dough chilled while batches are baking in the oven. Never place cookie dough on a hot cookie sheet.