Preheat oven to 350 degrees F. Line 2 large baking sheets with parchment paper; set aside.

Whisk together the flour, espresso powder, baking powder and salt in a medium bowl.

Beat the butter on medium speed until creamy. Add the sugars; beat until fluffy, about 3 minutes. Beat in the eggs, 1 at a time.

Stir the dry ingredients into the butter-sugar mixture with a wooden spoon or large rubber spatula. Stir in the oats and chocolate chips.

Working with a generous 2 tablespoons of dough each time, roll the dough into 2-inch balls. Place the balls on the prepared baking sheets, spacing them at least 2 inches apart. Sprinkle the tops with a pinch of fleur de sel.

Bake until the cookie edges turn golden brown, 22 to 25 minutes. Let the cookies cool on the baking sheets for 2 minutes. Transfer the cookies with a wire rack and let cool for at least 30 minutes.