Whisk flour, 3 tablespoons (12 grams) finely ground coffee beans, and salt in a bowl until combined.
In a large bowl using an electric hand-held or stand mixer fitted with a paddle attachment, beat butter, ¼ cup (53 grams) brown sugar, granulated sugar, and vanilla extract on medium speed until fluffy and light in color, about 2–3 minutes.
With the mixer running on low speed, slowly add in dry ingredients. Beat on low just until the flour has combined, scraping down the sides of the mixer as needed. The dough will be slightly crumbly. Remove the bowl from the stand mixer, and stir chopped dark chocolate bar into the dough with a silicone spatula. Press the dough together, so it’s less crumbly and forms into a solid dough.
Line a clean work surface with a large piece of plastic wrap. Pour the dough onto the plastic wrap. Using the plastic wrap, so the dough doesn’t stick to your hands, shape the dough into a log about 1.5 inches (3.8 cm) in diameter. You can roll the dough along the counter to help with this process. Be sure to push any large pieces of chocolate to the inside of the dough log, or it could mis-shape the cookies as they are baking.
Refrigerate the dough until firm, about 60–90 minutes.
Heat oven to 350°F (175°C).
Whisk egg in a small bowl. On a shallow plate mix the remaining 2 tablespoons (8 grams) finely ground coffee beans and the remaining ¼ cup (53 grams) brown sugar.
Remove the cookie dough log from the refrigerator, and brush the outside in egg wash using a pastry brush. Roll the outside of the log in the espresso sugar mixture.
Slice the log into ½ inch (13 mm) pieces, and place on a parchment lined baking sheet 1 inch (25 mm) apart. Garnish each cookie with Maldon sea salt flakes.
Bake on the upper and lower third oven racks, rotating halfway through baking, for 17–21 minutes, or until cookie edges have slightly browned and firmed up. Remove from the oven and immediately transfer to a wire cooling rack, and allow to cool completely.
Store cookies in an airtight container at room temperature for up to 5 days.