Place the butter and chocolate in a large microwave-safe bowl. Heat at half-power in 30-second increments until the butter has melted and the chocolate melts when stirred. Stir until smooth, then allow to cool slightly. (5-10 minutes)

Whisk in the cocoa powder, and then whisk in the sugar, followed by the eggs and vanilla. Finally, whisk in the flour and salt.

Transfer the batter to the prepared pan, and spread evenly. Bake 30 to 35 minutes, or until a pick inserted into the center comes out with a few moist crumbs.

Set the pan on a wire rack, and allow the brownies to cool completely. Then chill for 15 minutes before adding the peanut butter layer.

Using an electric mixer on medium speed, beat the peanut butter and butter until smooth and creamy.

Add the sugar, 1 tablespoon of the milk, vanilla, and salt, and mix until thoroughly combined and fluffy. The frosting should be thick but spreadable. (Add a bit more milk if necessary.)

Spread the peanut butter frosting over the cooled brownies. Place the pan in the refrigerator while you make the ganache. (Chill at least 20 minutes.)

Place the chocolate and corn syrup (if using) in a heatproof bowl, and set aside.

Heat the cream in a saucepan over medium heat just until it begins to bubble.

Pour the hot cream over the chocolate. Let sit for 5 minutes before whisking until smooth. Stir in 1 teaspoon butter.

Pour the ganache over the chilled peanut butter frosting on the brownies, taking care to cover evenly.

Refrigerate until the ganache has set before serving.