Preheat the oven to 350°F.

In a large bowl, whisk together melted butter, sugar, vanilla, and salt.

Add the eggs, one at a time, whisking well after each addition.

Whisk in the cocoa powder and flour, mixing until just blended.

Line a 9-inch square pan with foil, hanging it over the edges.

Spray foil with non-stick spray.

Spread batter evenly into the prepared pan.

Drizzle the peanut butter over the brownie batter and use a knife to lightly swirl.

In a small bowl, stir together flour, brown sugar, granulated sugar, peanut butter, and butter, until blended and crumbly.

Sprinkle the streusel evenly over the brownie batter.

Bake for 40-45 minutes, or until a toothpick inserted near the center comes out mostly clean, with a few moist crumbs.

Remove to a wire rack to cool completely.

Cut into bars.