1. Preheat oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper.

2. Melt butter and half the chocolate chips in a microwave-safe bowl, stirring until smooth, then let cool slightly.

3. Whisk eggs, sugar, and vanilla until slightly thickened, then fold in melted chocolate.

4. Sift cocoa powder, flour, and salt into the mixture and fold until just combined, then stir in remaining chocolate chips.

5. Pour batter into prepared pan and bake for 20 to 25 minutes, or until a toothpick inserted comes out with moist crumbs; cool completely.

6. In a chilled bowl, whip heavy cream, powdered sugar, and vanilla to soft peaks.

7. Gently stir one-third of whipped cream into melted chocolate to lighten, then fold in remaining cream until uniform.

8. Spread mousse evenly over cooled brownie base, smooth the top, and refrigerate for at least 2 to 3 hours until set.

9. Cut into servings, garnish with whipped cream, chocolate shavings, or fresh raspberries, and serve chilled or at room temperature.

10. Store in an airtight container in the refrigerator for up to 4 days.