Preheat the oven to 350ºF, and line an 8x8 baking pan with parchment paper. Spray with nonstick cooking spray.

In a medium bowl, whisk the flour, baking soda, and salt until combined; set aside.

In the bowl of an electric stand mixer fitted with a paddle attachment, cream the butter until smooth. Add both sugars and mix on high for 3 minutes until light and fluffy. Add the egg, followed by the vanilla. Then stir in the pudding mix.

With mixer set on low speed, slowly add the flour mixture into the wet mixture until everything has been incorporated. Stir in the white chocolate chips until just combined.

Press the cookie dough evenly into the prepared pan. Top with a single layer of Oreos.

In a medium saucepan over medium heat, melt the butter and cocoa powder together and stir with a wooden spoon until smooth and glossy. Remove from the heat, and stir in the sugar. Add the eggs and vanilla, stirring until fully incorporated. Stir in the flour and salt.

Pour batter on top of the Oreos, spreading out evenly. Sprinkle the Reese's cups, Reese's Pieces, and peanut butter chips on top. Bake for 35-40 minutes. Remove from the oven and allow to cool for at least 2 hours. Cut into squares and enjoy!