Adjust the oven rack to the 2nd level mark (just above center) and preheat your oven to 350ºF. Line two baking sheets with parchment paper or silicone mats and set aside.

In a medium bowl, whisk together the all-purpose flour, baking powder, and salt; set aside.

Using a hand mixer or stand mixer fitted with the paddle attachment, beat the eggs, egg yolk, granulated sugar, vegetable oil, and vanilla extract on medium-high speed until frothy and combined.

In a small saucepan, heat the semi-sweet chocolate chips and unsalted butter together over medium heat until melted and smooth. Remove from heat and whisk in the unsweetened cocoa powder and optional espresso powder.

Pour the melted chocolate mixture into the wet ingredients and mix until just combined.

Add the dry ingredients and mix on low speed until just combined; the batter will be thin but scoopable.

Using a 1 1/2 tablespoon-size cookie scoop or two spoons, scoop out the cookie dough onto the baking sheets, leaving 1 1/2 inches of space between them.

Sprinkle sea salt flakes over the top of each cookie and bake for 8-10 minutes or until tops are shiny and crackly.

Remove from the oven and top with additional chocolate chips if desired. Use a round cookie cutter to shape them if needed.

Let the cookies cool on the baking sheets for 5 minutes before transferring to a cooling rack.