Set oven to 350°F (175°C). Line a 9x13-inch pan with parchment paper, leaving extra on the sides for easy lifting.
In a large bowl, whisk flour, cocoa powder, and salt until evenly combined.
In another bowl, whisk hot melted butter, canola oil, granulated sugar, and brown sugar for 1 minute until glossy and smooth.
Whisk in eggs and vanilla for 2 minutes until the mixture is thick and shiny.
Fold the wet mixture into the dry ingredients just until no flour remains. Don’t overmix.
Spread half the batter into the pan. Dollop peanut butter over it, then smooth into an even layer.
Gently spoon the remaining batter over the peanut butter and spread to cover completely.
Bake for 40–45 minutes until edges are set and a toothpick comes out with moist crumbs. Don’t overbake.
Cool completely in the pan. Lift out using the parchment, transfer to a board, and cut into squares