In a large bowl, beat together the sugar and egg until pale in color. Whisk in the kahlua and vanilla extract to fully incorporate.
Add the cocoa, cornstarch, baking powder, espresso powder, and salt and whisk in until evenly combined.
Add the nutella and chocolate chips and fold in until fully incorporated and chocolate chips are evenly dispersed.
Cover dough with plastic wrap and chill for about 1 hour before baking.
Meanwhile, preheat oven to 350ºF with oven racks in bottom and top third positions. Line two baking sheets with parchment paper.
Shape chilled dough into 1 1/2 oz mounds, slightly taller than they are wide. Arrange 9 on each baking sheet, evenly spaced out. Sprinkle tops with fleur de sel if desired.
Bake cookies in preheated oven for 10-12 minutes, flipping and rotating halfway through, until centers are puffed up and look dry/set. Cool on pan for about 10 minutes, and then transfer to a wire rack to cool completely.
Cookies are best served fresh but can be stored for a few days in an airtight container at room temperature.