Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a medium bowl, combine the all-purpose flour, unsweetened cocoa powder, baking powder, kosher salt, and espresso powder. Whisk until evenly blended.

Melt the unsalted butter along with both granulated and light brown sugars in a microwave-safe bowl for about 30 seconds. Whisk until smooth.

Melt the semi-sweet or dark chocolate in a separate bowl and let cool slightly before combining with the butter and sugar mixture, then mix in eggs and vanilla.

Fold the dry ingredients into the wet mixture using a spatula until no dry flour remains. Avoid overmixing.

Portion and shape the dough into roughly 2-tablespoon-sized balls and place on prepared baking sheets, spacing them 2 inches apart.

Bake for 10 to 12 minutes, until slightly underbaked in the center.

Allow to cool on baking sheets for about 5 minutes before transferring to a wire rack. Sprinkle each cookie with a pinch of flaky salt before serving.