1. Preheat your oven to 350°F. Line a 9×13-inch baking dish with parchment paper.
2. In a large bowl, whisk together flour, cornstarch, cocoa powder, baking powder, and salt.
3. In a separate bowl, beat eggs, sugar, melted butter, and vanilla until light and fluffy.
4. Gradually add the dry ingredients to the wet mixture until well combined.
5. Pour the batter into the prepared baking dish, smooth the surface, and bake for 30-35 minutes, until the edges are set but the center remains moist. Cool completely on a wire rack.
6. (Optional Step) Bloom gelatin in 2 tablespoons of water, letting it sit for about 10 minutes before use.
7. Heat ½ cup of heavy cream and milk until steaming but not boiling. Stir in the chocolate until melted and smooth.
8. (Optional Step) If using, add the bloomed gelatin to this mixture and stir until dissolved.
9. Allow the chocolate mixture to cool to room temperature.
10. In a separate bowl, whip the remaining 1 cup of heavy cream with powdered sugar to stiff peaks.
11. Gently fold the cooled chocolate mixture into the whipped cream until combined.
12. Spread the mousse evenly over the cooled brownies. Garnish with chocolate curls if desired.
13. Refrigerate the mousse-topped brownies for at least 2 hours or up to overnight for the best set.