Preheat oven to 350°F. Line the bottom of two 8x8-inch square pans with parchment paper or foil. Spray with nonstick cooking spray.
In a medium bowl sift together the flour, cocoa powder, baking powder, and salt.
In a large bowl whisk sugar, melted butter, and vanilla together. Whisk in eggs, one at a time, until combined. Stir in flour mixture until batter is smooth. Divide batter evenly among prepared pans, smoothing tops.
Bake for 20 minutes, or until brownies begin to pull away from the sides of pans. Transfer to wire cooling racks and let cool in pans completely. Can be made 1 day ahead of time and refrigerated.
For the mousse:
Bring the cream to a simmer in a medium saucepan. Remove from heat and add chocolate. Let mixture stand for 1 minute then whisk until chocolate is melted and mixture is smooth. Pour into a medium bowl and refrigerate for about 30 minutes, or until chocolate is cool and barely set.
With an electric mixer fitted with a whisk attachment, whisk the cooled chocolate mixture until lightened in color and fluffy. Do not overbeat.
For the whipped cream:
In a clean medium bowl, whip the cream, sugar, and vanilla with an electric mixer fitted with a clean whisk attachment and set to high speed for about 2 minutes, or until cream is stiff.
To assemble:
Brush 1 brownie layer with 2 tablespoons of the liqueur. Spread half the chocolate mousse mixture onto brownie in the pan. Top with a thin layer of the whipped cream. Carefully lift second brownie out of pan and place on top of whipped cream layer. Repeat steps with remaining brownie, liqueur, mousse, and whipped cream. Cover and refrigerate until chilled, at least 2 hours and up to 1 day.
Remove from the refrigerator 15 minutes before serving. Cut into 9 squares.