Preheat oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper.

In a microwave-safe bowl, melt the butter and semi-sweet chocolate together in 30-second intervals, stirring between each until smooth.

In a large bowl, whisk together the granulated sugar, light brown sugar, eggs, and vanilla extract until well combined.

Add the melted chocolate mixture to the sugar mixture and whisk until smooth.

Sift in the flour, cocoa powder, and salt, and fold until just combined.

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

In a separate bowl, mix the peanut butter and powdered sugar until smooth. Drop spoonfuls of the peanut butter mixture over the brownie batter.

Pour the remaining brownie batter over the peanut butter layer and gently spread it to cover.

Bake for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

While the brownies are baking, heat the heavy cream in a saucepan over medium heat until it begins to simmer. Remove from heat and add the dark chocolate. Let sit for 5 minutes, then stir until smooth to create a ganache.

Once the brownies are baked and slightly cooled, pour the ganache over the top, spreading it evenly.

Let the ganache set before slicing into bars.