Preheat your oven to 350°F (175°C). Grease a 13×9-inch baking pan.

Melt the butter and chopped semisweet chocolate together over low heat in a heavy saucepan, stirring until smooth. Let cool slightly.

In a large mixing bowl, beat the eggs and sugar on low speed until combined. Increase to medium-high and beat for about 5 minutes until thick and pale.

Mix in the vanilla extract followed by the cooled chocolate mixture.

Fold in the flour and salt just until incorporated. Stir in walnuts if using.

Spread the batter evenly into the prepared pan.

Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean.

Cool for about 15 minutes on a wire rack.

To prepare the hot fudge sauce, combine the cream, sugar, and butter in a saucepan. Heat until the sugar dissolves, then bring to a gentle boil without stirring. Reduce heat and simmer for 2 minutes.

Place the cocoa powder in a heatproof bowl and carefully whisk in the hot cream mixture until smooth.

Cut the brownies into squares. To assemble your Brownie Sundae Recipe, top each brownie with vanilla ice cream, warm hot fudge sauce, whipped cream, and a maraschino cherry if desired.