For the brownies:
2 sticks unsalted butter, melted
2 cups granulated sugar
2 cups unsweetened natural cocoa powder
1/2 teaspoon salt
1 teaspoon vanilla
4 large eggs
1 cup all-purpose flour

For the filling:
2 tablespoons unsalted butter
5 ounces semisweet chocolate, chopped
1 teaspoon vanilla extract
1/2 cup granulated sugar
3 tablespoons cocoa powder
3 tablespoons cornstarch
1/4 teaspoon salt
3 large egg yolks
1 cup heavy cream, divided
2 cups whole milk

For the topping:
1 cup heavy cream
1/4 cup powdered sugar
1/2 teaspoon vanilla extract