Preheat the oven to 350°F. Line light colored cookie sheets with parchment paper or a silicone baking mat and set aside.
Using a stand mixer fitted with the paddle attachment or a handheld electric mixer and a large bowl, beat together the melted 3/4 cup unsalted butter, 1 cup light brown sugar and 1/4 cup sugar for 2-3 minutes. Add the 1 egg, 1 egg yolk and 2 teaspoons vanilla extract. Beat until combined.
In a medium bowl, whisk or sift together the 1 3/4 cups flour, 3 tablespoons espresso powder and 1 teaspoon baking soda.
Add the dry ingredients to the wet mixture and mix together until the cookie dough is formed, scraping down the sides of the bowl with a spatula occasionally. Do not over-mix.
Fold in the chopped 1 cup chocolate chunks or chocolate chips by hand.
With a large cookie scoop or ice cream scoop, equivalent to 1/4 cup, scoop cookie dough into balls and place on the prepared cookie sheets. *Optional: press extra chopped chocolate on top.
Bake for 9-11 minutes. Remove the cookies from the oven and top with flaky sea salt (optional). Allow the cookies to cool the cookie sheets for a few minutes before transferring to a cooling rack.
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Notes
Storage: Once completely cool, keep the cookies in an airtight container at room temperature. For the best texture, enjoy within 3–4 days.Freezing baked cookies: Freeze completely cooled cookies in an airtight freezer container, with parchment between layers, for up to 3 months for best quality. Thaw covered at room temperature.Freezing cookie dough: Freeze individual scoops on a lined tray, then transfer them to a freezer bag or airtight container. Use within 3 months for best quality. Bake from frozen, allowing additional time and checking for set edges and soft centers.