1. Preheat oven to 350°F (175°C). Line a 9×13 inch pan with parchment, leaving overhang; lightly grease.

2. Melt butter, then stir in granulated sugar and brown sugar until dissolved; remove from heat.

3. Whisk in eggs one at a time, then stir in vanilla.

4. Whisk flour, cocoa powder, and salt together in a separate bowl.

5. Fold dry ingredients into wet just until combined; fold in chocolate chips if using.

6. Spread batter evenly in pan and bake 25–30 minutes, until a toothpick shows moist crumbs.

7. Cool the brownie base completely in the pan on a wire rack.

8. Melt chopped chocolate gently (double boiler or short microwave bursts); let cool slightly.

9. Whip cold heavy cream to soft peaks; add optional sugar and vanilla, and whip to medium-firm peaks.

10. Fold a spoonful of whipped cream into melted chocolate to lighten, then gently fold in the remaining cream with a pinch of salt until uniform.

11. Spread chocolate mousse over the fully cooled brownie base.

12. Cover and refrigerate 2–4 hours or overnight until the mousse is set.

13. Lift out using parchment, slice with a hot, wiped-clean knife, and serve.