Make the Brownie Layer:

Preheat oven to 350°F (175°C).

Line an 8x8-inch or 9x9-inch baking pan with parchment paper or grease lightly.

In a bowl, whisk together melted butter and sugar until combined.

Add eggs and vanilla, mixing until smooth and glossy.

Stir in cocoa powder, flour, salt, and chocolate chips (if using).

Spread batter evenly into the prepared pan and bake for 20–25 minutes, or until a toothpick inserted comes out with moist crumbs.

Let cool completely before adding the mousse.

Make the Chocolate Mousse:

In a microwave-safe bowl, melt chocolate chips with ½ cup of heavy cream in 20-second intervals, stirring each time until smooth. Let cool to room temperature.

In a separate bowl, whip the remaining 1 cup of cold heavy cream until soft peaks form. Add vanilla and powdered sugar (if using) and continue to whip until stiff peaks form.

Gently fold the cooled chocolate mixture into the whipped cream until fully combined and fluffy.

Assemble & Chill:

Spread the mousse evenly over the cooled brownie base.

Refrigerate for 2–3 hours, or until mousse is set.

Slice & Serve:

Top with whipped cream, chocolate shavings, or a dusting of cocoa powder.

Slice into squares using a sharp knife wiped clean between cuts.