For the cream cheese mixture: Beat the cream cheese, butter, peanut butter and confectioners' sugar together with an electric mixer. Set aside until ready to use.
For the cake: Preheat the oven to 350 degrees F. Spray a 13- by 9- by 2-inch cake pan with nonstick spray.
Mix the cake mix, milk, vegetable oil and eggs together. Beat for about 2 minutes using an electric mixer. Fold in the chopped candy. Line the bottom of the prepared pan with the chopped peanuts and pour the batter into the pan.
Evenly space dollops of the cream cheese mixture on top of the cake. When the cake cooks, the cream will sink down to the bottom. This will be your "volcano" of flavor. Place in the oven for 35 to 45 minutes.
For the glaze: In a small saucepan, melt 4 tablespoons of the butter over medium-low heat. Whisk in the cocoa powder and add the milk until you get a thin consistency. If the glaze is too thick, add more milk. Add the remaining butter and bring the butter and milk mixture to a boil. Whisk in the confectioners' sugar and vanilla extract; the glaze should be thin. Remove from the heat and allow to cool slightly before using.
While the cake is still hot, spread the glaze evenly over the cake.