Preheat oven to 350° F. Grease a 9-by-13-inch half-sheet pan then cover the bottom and sides with parchment paper for easy brownie extraction and clean-up.
Using an electric mixer on medium-low speed, mix the brown sugar and butter in a large bowl until blended but not fluffy, about 2-3 minutes.
Add the eggs one at a time, mixing at medium speed after each addition and scraping down the sides of the bowl occasionally. Mix in the vanilla and salt. Turn the mixer to medium-high speed and beat for 30 seconds.
In a medium bowl, sift (seriously-it's annoying but it matters) together the flour, cocoa powder, and espresso powder. Pour into the wet batter and mix on low speed, scraping down the sides and bottom of the bowl often, until just mixed. Fold in half the chocolate chunks.
Pour the batter into the prepared pan, then sprinkle the top evenly-ish with the remaining chocolate chunks.
Bake for 47-52 minutes (this time will really depend on your oven). Because of the chocolate, a toothpick test won't help here. The batter should show the barest hint of a wobble in the middle of the pan. Allow the brownies to cool overnight or chill in the freezer after they've cooled to room temperature. These need the overnight rest for the maximum flavor to shine through.