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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. Set to the side.
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In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set to the side.
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Using a hand mixer or stand mixer fitted with the paddle attachment, beat the eggs, egg yolk, granulated sugar, vegetable oil, and vanilla extract together on medium-high speed until fully combined and frothy.
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Meanwhile, in a small saucepan, heat the semi-sweet chocolate chips and unsalted butter together over medium heat until fully melted and smooth. Remove from heat and immediately whisk or stir in the unsweetened cocoa powder and espresso powder.
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Carefully pour the melted chocolate mixture into the wet ingredients and mix until the chocolate is just combined.
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Add in the whisked dry ingredients and mix on low speed until just combined. You can also simply fold in the dry ingredients using a spatula. The batter will be on the thinner side, but still scoopable.
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Using a 1 ½ tablespoon-size cookie scoop or 2 spoons, scoop the cookie dough out and place onto the baking sheets, leaving about 1 ½ inches of space between them.
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Sprinkle some sea salt flakes over the top of each cookie, then bake for 8-10 minutes at 350ºF or until the tops of the cookies have a shiny, crackly top to them.
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Remove from the oven and top with a few additional chocolate chips (optional). You can also choose to use a round cookie cutter to shape them into perfect round cookies. Do this immediately after they have come out of the oven while they’re still soft. Visit my ‘how do you make perfectly round cookies?’ section of the blog post for all my cookie shaping tips.
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Let the cookies cool on the baking sheets for about 5 minutes before transferring them to a cooling rack to cool almost completely—because they are really good still warm! Use a spatula to transfer the cookies because they will be very soft!
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Cookies are best kept in a sealed food container at room temperature for up to 2 days.