Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan.
In a large mixing bowl, melt the butter. Stir in the granulated sugar and brown sugar until well combined.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, salt, and baking powder.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. Fold in the chocolate chips.
Pour the brownie batter into the prepared baking pan and spread it evenly. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool completely in the pan.
While the brownies are cooling, prepare the mousse. In a small saucepan, melt 1/2 cup of chocolate chips over low heat, stirring until smooth. Remove from heat and let it cool slightly.
In a mixing bowl, whip the heavy cream and vanilla extract until soft peaks form. Gently fold the melted chocolate into the whipped cream until fully combined.
Once the brownies are cool, spread the chocolate mousse evenly over the top. Refrigerate for at least 2 hours to set the mousse.
Cut into squares and serve chilled.