Heat butter and sugar over medium-low, stirring often until melted and smooth. Remove from heat and let cool. You can do this step in the microwave if that’s easier. Just microwave on high and stir every 30 seconds until melted.
In a mixing bowl, whisk together the eggs and sugar, then stir in the flour.
Stir the salt, vanilla and the slightly cooled melted chocolate mixture into the egg mixture until thoroughly blended. Pour into the partially baked crust and bake at 350 for about 25 minutes. The filling should still be a little big jiggly in the center. Let cool at room temperature.
Spread softened ice cream over brownie filling and freeze until firm. Cover with plastic wrap and freeze until firm.
Before serving, cover the pie with sweetened whip cream or whipped topping.