1. Preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving 2-inch overhangs on short sides. Lightly grease the parchment.

2. Chop dark chocolate into small pieces. Melt butter and chocolate together in a double boiler or microwave, stirring until completely smooth. Let cool for 5 minutes.

3. In a large bowl, whisk sugar into the melted chocolate mixture until well combined.

4. Add eggs one at a time, whisking vigorously after each addition until batter becomes thick and glossy. Mix in vanilla extract.

5. In a separate bowl, sift together flour, cocoa powder, and salt. Add espresso powder if using.

6. Fold dry ingredients into chocolate mixture using a rubber spatula, stirring just until no flour streaks remain. Do not overmix.

7. Pour batter into prepared pan and spread evenly with an offset spatula. Tap pan on counter several times to release air bubbles.

8. Bake for 28-32 minutes until top looks set with slight sheen and a toothpick inserted in center comes out with moist crumbs (not wet batter, but not clean).

9. Cool brownies in pan on wire rack for at least 20 minutes, or cool completely for cleanest cuts.

10. Use parchment handles to lift brownies from pan. Cut into 12 large squares (3×3 inches) using a sharp knife wiped clean between cuts.

11. To assemble sundae: Place one brownie square in serving bowl. Warm in microwave for 10-15 seconds if desired.

12. Add 2-3 scoops vanilla ice cream on top of warm brownie.

13. Drizzle generously with hot fudge sauce.

14. Top with whipped cream, sprinkle with chopped nuts, add chocolate shavings if using, and crown with a maraschino cherry.

15. Serve immediately with a spoon and enjoy the temperature and texture contrasts!