In a medium saucepan set over medium heat, melt the butter. Add the sugar and salt and stir until combined. Remove from heat.
In a small bowl, whisk the cornstarch with about 1/4 of the milk until very smooth. Whisk in the eggs. Pour the remaining milk into the butter mixture, whisking constantly. Whisk in the cornstarch mixture.
Return the pan to medium heat and bring to a boil, whisking constantly. Once it begins to boil, reduce heat to a simmer and cook for one minute, whisking constantly, until the mixture thickens. Remove from heat and stir in the vanilla. Cover and refrigerate until thoroughly chilled, at least 4 hours.
Cut the brownies into cubes. Refrigerate until ready to use.
In the bowl of an electric mixer fitted with the whisk attachment, beat the cream until frothy. Add the powdered sugar and vanilla and continue to beat on high speed until firm peaks form.
In individual trifle dishes or a large trifle dish, cover the bottom with a layer of brownie cubes. Spoon a layer of butterscotch pudding over then top with a layer of whipped cream. Continue until the layers reach the top of the dish. Serve.