Preheat oven to 350°F and grease a 9"-x-13" baking pan with cooking spray.
In a large mixing bowl, beat together the sugar and unsalted butter to get a light and fluffy consistency. Then mix in the vanilla extract.
In a separate bowl, combine the dry ingredients and whisk together. Slowly add it into the mixture and combine until it forms into a smooth ball of dough. Press the dough evenly into the baking pan. Set aside in a cool place.
Next, you will prepare the filling. In a mixing bowl, beat together the cream cheese and melted unsalted butter. Then add the vanilla extract, eggs, and salt and mix well.
In another bowl, add the powdered sugar and whisk to remove any clumps. Slowly add the powdered sugar into the filling mixture and beat until it is silky smooth. Set aside.
Spread a thin layer of peanut butter over the rested dough in the baking pan. Arrange Reese's Peanut Butter Cups 4 x 4 top side down, over the peanut butter, and gently press them into the layer. Pour the filling over the Reese's Peanut Butter Cups and spread it across the pan evenly.
Bake at 350°F for 40 minutes or until the cake is golden brown, puffed, and slightly cracked. The middle of the cake should be GOOEY! If it is not gooey, you have overbaked the cake.
Remove from the oven, cool completely, cut into desired squares, sprinkle with powdered sugar, and serve.