Step 1: Prepare for Baking

1. Preheat your oven to 350°F (175°C).
2. Line a 9×13-inch baking pan with parchment paper, leaving overhang on the sides for easy removal.
3. Spray the parchment paper lightly with cooking spray.

Step 2: Make the Brownie Layer

1. In a medium microwave-safe bowl, melt the butter in 30-second intervals until completely melted.
2. Add the granulated sugar to the melted butter and whisk until well combined.
3. Add the eggs one at a time, whisking well after each addition.
4. Stir in the vanilla extract.
5. In a separate bowl, sift together the cocoa powder, flour, and salt.
6. Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
7. Fold in the chocolate chips.
8. Pour the brownie batter into the prepared pan and spread evenly.
9. Bake for 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).
10. Allow the brownies to cool completely in the pan on a wire rack.

Step 3: Prepare the Chocolate Mousse

1. In a small bowl, sprinkle the gelatin over the cold water and let it bloom for 5 minutes.
2. In a heatproof bowl, combine the chocolate chips and 1/2 cup of heavy cream.
3. Microwave in 30-second intervals, stirring after each, until the chocolate is melted and smooth.
4. Heat the bloomed gelatin in the microwave for 10 seconds until liquified, then stir into the chocolate mixture.
5. Allow the chocolate mixture to cool to room temperature (but not set).
6. In a large bowl, whip the remaining 1 1/2 cups heavy cream with the sugar and vanilla until soft peaks form.
7. Gently fold the whipped cream into the cooled chocolate mixture until no streaks remain.

Step 4: Assemble the Layers

1. Pour the chocolate mousse over the cooled brownie layer, spreading evenly with an offset spatula.
2. Cover with plastic wrap (touching the surface of the mousse to prevent a skin from forming).
3. Refrigerate for at least 4 hours, preferably overnight, until the mousse is set.

Step 5: Add Optional Topping and Serve

1. Before serving, whip the heavy cream with powdered sugar until stiff peaks form.
2. Spread or pipe the whipped cream over the set mousse layer.
3. Garnish with chocolate shavings or a light dusting of cocoa powder.
4. Use the parchment paper overhang to lift the entire dessert out of the pan.
5. Cut into squares with a sharp knife, wiping the blade clean between cuts for neat edges.