Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.

In a medium saucepan, melt the butter and bittersweet chocolate together over low heat, stirring until smooth.

Remove from heat and stir in the granulated sugar, mixing well.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

Fold in the flour and salt until just combined.

Pour half of the brownie batter into the prepared pan, smoothing it evenly.

In a separate bowl, combine peanut butter and powdered sugar until smooth.

Spread the peanut butter mixture over the brownie batter in the pan.

Top with the remaining brownie batter, spreading it gently over the peanut butter layer.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted comes out with moist crumbs.

For the topping, melt the semi-sweet chocolate and vegetable oil together until smooth. Pour the melted chocolate over the cooled brownies and spread evenly.

Let the brownies cool completely before cutting into squares and serving.