Line an 8-inch round bowl with plastic wrap, leaving an overhang. Press the softened vanilla ice cream evenly into the bowl, smooth the top, cover and freeze for at least 6 hours.
Heat the oven to 350 F and line the bottom of an 8-inch round cake pan with parchment paper. Melt the bittersweet chocolate and butter together over low heat, stirring until smooth, then cool for 5 minutes.
Whisk the granulated sugar, eggs and vanilla into the chocolate mixture. Fold in the flour, cocoa powder and salt just until no dry streaks remain.
Spread the brownie batter in the prepared pan and bake for 20 to 24 minutes, until the edges are set and the center has moist crumbs. Cool completely, then remove the brownie from the pan.
Place the brownie on a freezer-safe serving plate. Unmold the frozen ice cream dome onto the brownie, remove the plastic wrap and return the dessert to the freezer.
Set the egg whites, sugar and cream of tartar over a saucepan of gently simmering water. Whisk constantly until the sugar dissolves and the mixture reaches 160 F, about 4 to 6 minutes.
Beat the warm egg white mixture until glossy, thick peaks form and the bowl feels cool. Beat in the vanilla and salt, then cover the ice cream and exposed brownie completely with a thick layer of meringue.
Brown the meringue with a kitchen torch, or place the dessert under a fully heated broiler for 1 to 2 minutes while watching constantly. Serve immediately or freeze for up to 30 minutes before serving.