Preheat oven to 350F, line two baking sheets with parchment paper.
Melt the 1 1/3 cups chocolate chips with butter until smooth, let cool slightly.
Whisk sugar, eggs, espresso powder, and salt for about a minute until lighter in color.
Pour in the cooled chocolate mixture and whisk until combined.
Sift in flour, cornstarch, and baking soda, mix until smooth, fold in remaining chocolate chips.
Let dough rest a couple minutes, then scoop 12 portions onto baking sheets, spaced 3 inches apart.
Bake 12-13 minutes, rotating pans halfway, until tops are shiny and cracked. Top with flaky salt.
Cool on pan 10 minutes, then move to a wire rack.