• Preheat oven to 350°F. Line an 8×8-inch square baking pan with parchment paper or foil. If using foil, spray with cooking spray.
• In a medium bowl, whisk together the flour, cocoa powder, and salt. Set aside.
• In a large mixing bowl, beat together the butter, sugar, and vanilla until creamy. Add the eggs one at a time, beating after each.
• Stir the dry ingredients into the batter until combined. Pour into the prepared pan and spread evenly. Sprinkle with chocolate chips.
• Bake for 22–25 minutes.
• While the brownie is baking, combine the peanut butter and canola oil in a microwave-safe bowl and microwave on high for 20 seconds to soften.
• Remove the brownie from the oven. Pour the warm peanut butter mixture evenly over the hot brownie layer and spread gently.
• Sprinkle the marshmallows over the peanut butter layer. Return to the oven for 3 more minutes, until the marshmallows puff.
• Remove from the oven and sprinkle immediately with the toasted pecans.

Make the frosting

• In a small saucepan, melt the butter over medium heat. Add the cocoa powder and 1/4 cup of the milk and heat, stirring, until the mixture just begins to boil.
• Remove from heat. Pour into a mixing bowl with the sifted confectioners’ sugar and a pinch of salt. Beat on high speed with an electric mixer until smooth and glossy.
• If the frosting is not pourable, add the remaining 2 tablespoons of milk and beat again.
• Beat in the vanilla extract. Pour the frosting evenly over the puffed marshmallow and pecan layer.
• Refrigerate for 1–2 hours until completely set. Cut into squares and serve.