In the bowl of a stand mixer fitted with the paddle attachment, beat butter, sugars, vanilla bean paste, and espresso powder on medium-high speed until light and fluffy, about 3-5 minutes.
Use a spatula to scrape down side of bowl. With the mixer on low speed, slowly add the flour. Next add the chocolate chunks and mix until just combined.
Divide the dough in half and shape each half into a log, about 2-2 ½ inches in diameter. Tightly wrap the logs in plastic wrap and chill in the refrigerator for at least 2 hours (see notes)*.
Preheat oven to 350 degrees F. Line two baking sheets with parchment paper. Unwrap the chilled logs and brush with egg wash. Roll the logs in demerara sugar.
Slice each log into ½-inch thick rounds. Place on prepared baking sheet, about 1 inch apart. Sprinkle with sea salt and bake until edges are just beginning to brown, about 12-15 minutes.
Let cool slightly on a cooling rack before enjoying!
Store cookies in an airtight container.