We garnished these brownies with decorative chocolate squares. You can also garnish them with chocolate shavings.
Make brownie: Preheat oven to 325 degrees. Butter a 9-inch square pan (preferably springform). Sift flour, baking powder, and salt into a small bowl; set aside. Melt butter and chocolate, stirring, in a heatproof bowl set over a pan of simmering water. Let cool slightly.
Beat egg and yolk in the bowl of an electric mixer fitted with the whisk attachment until pale and thick, about 3 minutes. Crumble in brown sugar; beat until sugar is dissolved and mixture is thick, about 3 minutes. Beat in vanilla.
Remove bowl from mixer; fold in flour mixture and melted chocolate in alternating batches, beginning and ending with flour mixture. Pour batter into prepared pan; bake until center is just firm to the touch, about 15 minutes. Let cool completely in pan on a wire rack.
Make semifreddo: Heat milk in a small saucepan until just about to boil. Remove from heat. Whisk egg yolks, 1 tablespoon granulated sugar, and salt in a heatproof bowl. Slowly whisk in hot milk; place bowl over a pan of simmering water. Whisk constantly until pale and thick, about 5 minutes. Let cool completely, then chill custard until cold, about 15 minutes.
Melt white chocolate, stirring, in a heatproof bowl set over a pan of simmering water. Remove bowl from heat; let cool slightly. Gradually whisk in 1/2 cup custard; set aside.
Beat cream and remaining granulated sugar until soft peaks form. Fold cream into white-chocolate mixture in 2 additions. Pour over brownies. Freeze until firm, about 4 hours. Just before serving, cut into 36 1 1/2-inch squares.