Preheat oven to 350F with the rack in the middle. Line an 8×8 metal baking pan by lining it with aluminum foil, leaving a few inches of overhang on the sides to allow for easy removal.
Grease the foil and set aside. Add coconut oil and milk chocolate chips to a small saucepan set over low heat. Stir until the oil and chocolate have melted completely.
Remove from the heat and set aside to cool slightly. In the bowl of a stand mixer fitted with the whisk attachment, beat together eggs, sugar, and chocolate extract until smooth and pale, about 2 minutes.
With the mixer on low, add the melted chocolate mixture and mix until well combined.
Shut off the mixer and sift in the cocoa powder, cornstarch, espresso powder, and salt, mixing until well combined and scraping down the sides of the bowl as necessary.
Remove the bowl from the mixer and add in the coconut (if adding) and semisweet chips. The batter should be thick and smooth. Pour batter into the prepared pan, spreading it out evenly. Bake for 25-30 minutes or until the center is set and a toothpick inserted in the middle comes out clean. Do not overbake. Remove from oven and let cool in pan for about 15 minutes before removing and transferring to a wire rack to cool completely.
Store in an airtight container. I prefer to keep mine in the fridge but you can keep them at room temperature.