Whisk together cocoa powder, corn starch, and salt in a heat-proof bowl. Gradually add heavy cream, whisking until smooth, then incorporate egg yolks.

Heat milk and brown sugar in a saucepan until simmering. Mix hot milk into cocoa mixture, return to saucepan, and whisk until boiling. Cook for 2 minutes, then stir in vanilla and bittersweet chocolate.

Let the pudding cool slightly. Pour into a shallow pan, cover with wax paper, and chill in the refrigerator for at least 90 minutes.

Preheat oven to 350°F (175°C) and grease a pie dish. Melt semi-sweet chocolate and salted butter in a microwave-safe bowl in 30-second intervals, stirring until smooth.

Whisk in granulated sugar, salt, eggs, and vanilla extract until fully incorporated. Fold in all-purpose flour until no dry spots remain.

Pour brownie batter into the prepared pie dish and bake for about 20 minutes, or until a toothpick comes out with fudgy crumbs. Let cool completely on a wire rack.

Beat heavy cream with confectioners sugar and vanilla extract until stiff peaks form. Chill until ready to assemble the pie.

Layer chilled pudding over the brownie crust, then dollop whipped cream on top. Garnish with chocolate curls before serving.

Cover pie and refrigerate for at least one hour before serving to allow flavors to meld.