Preheat the oven to 350ºF. Line two baking sheets with parchment paper.
Whisk flour, baking powder, and salt in a medium bowl. Set aside.
In a mixer, combine eggs, egg yolk, sugar, oil, and vanilla on medium-high until frothy.
Melt butter and chocolate chips over medium heat, stirring until smooth. Remove from heat, stir in cocoa and espresso powder.
Combine melted chocolate with egg mixture. Fold in dry ingredients until just mixed.
Scoop batter onto prepared sheets, spacing cookies 1 ½ inches apart.
Sprinkle sea salt flakes on each cookie. Bake 8-10 minutes until shiny and crackly.
Optional: Top with additional chocolate chips and use a cookie cutter for round shapes.
Allow to cool on the sheet for 5 minutes before transferring to a rack.