Preheat the oven to 350°F. Line a 9×13-inch baking dish with parchment paper, leaving some overhang for easy removal of the brownies.

In a large bowl, whisk together flour, cornstarch, cocoa powder, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, add the eggs, sugar, butter, and vanilla. Beat on medium speed until light and fluffy.

With the mixer off, add the flour mixture. Turn the mixer on low and mix until well combined.

Pour the brownie batter into the prepared baking dish and bake for 30-35 minutes, or until the edges appear set and the center is still shiny or moist.

Let the brownies cool at room temperature.

In a microwave-safe bowl, heat ½ cup of the heavy cream for 50 seconds, then add the chocolate chips and stir until melted. Set aside to cool.

Pour the remaining 1 cup of heavy cream into a medium bowl and beat until soft peaks form, then continue whipping until stiff peaks form.

Fold the cooled chocolate mixture into the whipped cream until combined.

Pour the mousse over the cooled brownies and spread into an even layer. Garnish with chocolate curls if desired.

Refrigerate for 2 hours (up to overnight) before serving.