Adjust your oven rack to the second level from the bottom (just above center) and preheat the oven to 350ºF (175ºC). Line two baking sheets with parchment paper or silicone baking mats and set aside. In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until well combined, then set aside. In a separate large mixing bowl, use a hand mixer or stand mixer with the paddle attachment to beat the eggs, egg yolk, granulated sugar, vegetable oil, and vanilla extract on medium-high speed until the mixture is frothy and fully combined. While mixing, gently heat the semi-sweet chocolate chips and unsalted butter in a small saucepan over medium heat, stirring frequently until completely melted and smooth. Remove from heat and immediately whisk in the unsweetened cocoa powder and espresso powder (if using) until fully incorporated. Slowly pour the melted chocolate mixture into the wet ingredients and mix until just combined. Next, add the dry ingredients to the wet mixture and fold them in gently using a spatula or mix on low speed just until the batter comes together. The batter will be somewhat thin but still scoopable. Using a 1 ½ tablespoon cookie scoop or two spoons, drop rounded portions of dough onto the prepared baking sheets, spacing them about 1 ½ inches apart. Lightly sprinkle each cookie with sea salt flakes for a perfect sweet and salty balance. Bake the cookies at 350ºF for 8 to 10 minutes, or until the tops develop a shiny, crackled appearance. Immediately after removing the cookies from the oven, optionally press a few extra chocolate chips on top or use a round cookie cutter to gently shape the warm cookies into perfect rounds—check out the cookie shaping tips on my blog for more details. Allow the cookies to cool on the baking sheets for about 5 minutes to set, then carefully transfer them to a wire rack to cool almost completely. They are especially delicious when enjoyed warm, so use a spatula to move them gently as they will be soft. Store any leftovers in an airtight container at room temperature for up to 2 days for optimal freshness.