1- For the brownie layers, preheat the oven to 350ºF. Lightly butter two 8-inch square baking pans. Line the bottoms of the pans with parchment or waxed paper.
2- Whisk the sugar, melted butter, and vanilla together in a large bowl. One at a time, whisk in the eggs. Sift the flour, cocoa, baking powder, and salt together. Add to the butter mixture and stir until smooth. Divide the batter evenly among the pans and smooth the tops.
3- Bake until the brownies are beginning to pull away from sides, about 20 minutes. Transfer to wire cake racks and let cool in the pans. Invert and unmold onto the racks and remove the parchment paper.
4- For the mousse, bring the cream to a simmer in a medium saucepan. Remove from the heat. Add the chocolate and let stand until the chocolate softens, about 1 minute. Whisk until smooth. Pour into a medium bowl. Refrigerate until the chocolate mixture is cool and barely beginning to set, about 30 minutes.
5- For the whipped cream, whip the cream, confectioners' sugar, and vanilla in a chilled medium bowl with an electric mixer set on high until the cream is stiff.
6- Place 1 brownie layer in a clean 8x8 inch baking pan. Brush with 2 tablespoons of liqueur. Whisk the cooled chocolate mixture just until lightened in color and fluffy. Do not overbeat the mousse or it may separate. Spread half of the chocolate mousse on the brownie in the pan, and spread with a thin layer of the whipped cream. cover and refrigerate until chilled, at least 2 hours. (The dessert can be refrigerated for up to one day.) Remove from the refrigerator about 15 minutes before serving.
7- To serve, cut into 9 squares. Place a square in a shallow bowl and drizzle with as much chocolate sauce as you like. Serve chilled.