For the brownies:
- 2 sticks (226 grams) unsalted butter, melted
- 2 cups (400 grams) granulated sugar
- 2 cups (200 grams) unsweetened natural cocoa powder
- 1/2 teaspoon salt
- 1 teaspoon vanilla
- 4 large eggs
- 1 cup (127 grams) all-purpose flour
For the filling:
- 2 tablespoons unsalted butter
- 5 ounces (142 grams) semisweet chocolate, chopped
- 1 teaspoon vanilla extract
- 1/2 cup (100 grams) granulated sugar
- 3 tablespoons cocoa powder
- 3 tablespoons cornstarch
- 1/4 teaspoon salt
- 3 large egg yolks
- 1 cup (237 ml) heavy cream, divided
- 2 cups (473 ml) whole milk
For the topping:
- 1 cup (237 ml) heavy cream
- 1/4 cup (31 grams) powdered sugar
- 1/2 teaspoon vanilla extract
Make the brownies:
Preheat the oven to 325°F. Line a 13 by 9-inch pan with foil, leaving an overhang.
In a large bowl combine the butter, sugar, cocoa powder, and salt. Stir until well combined. Let cool if the butter is still hot. Stir in the vanilla and then the eggs, one at a time, stirring very well after each addition. Stir until the batter is very well blended. Add the flour and stir again until very well blended. Pour into prepared pan.
Bake for 25 minutes. Let cool to room temperature then place in the fridge to chill.
Make the filling:
In a large bowl, place the butter, chocolate, and vanilla.
In a medium saucepan, combine the sugar, cocoa powder, cornstarch, and salt. Gradually whisk in 1/2 cup of the cream, breaking up lumps with vigorous whisking. Whisk in the egg yolks.
Place the saucepan over medium heat and gradually whisk in the remaining cream and the milk. Bring to a boil, whisking constantly, until the mixture thickens. Boil for 1 minute.
Remove from heat and pour the hot milk mixture over the butter mixture. Whisk until the butter and chocolate are melted and the mixture is smooth. Pour the mixture through a strainer and over the chilled brownies. Press plastic wrap against the surface of the filling and return to the fridge to chill for at least 2 hours.
Make the topping:
In the bowl of an electric mixer fitted with the whisk attachment, beat the cream for 30 seconds. Add the sugar and vanilla and beat until the cream forms soft peaks. Gently spread the whipped cream over the brownies. Cut into squares and serve.