In a heatproof bowl, combine chocolate chips, 4 tablespoons butter, half of the marshmallow cream (3 1/2 ounces), and the vanilla extract. Set aside.
In another heatproof bowl, combine peanut butter, 4 tablespoons butter, and half of the marshmallow cream (3 1/2 ounces). Set aside.
In a large (see note) saucepan, combine evaporated milk and sugar. Stir until well mixed.
Heat to a rolling boil (see note) and continue to boil for 6 minutes, stirring continuously.
Pour into a 4 or 6 cup glass measuring cup (it will get HOT!) and pour half of the mixture into the bowl with the chocolate chips, and the remaining half into the bowl with the peanut butter.
Mix each mixture thoroughly until everything is melted, smooth, and well combined. This is most easily done if you have someone helping you or if you switch off mixing between the peanut butter and chocolate mixtures.
In a lined 9x13 baking sheet, pour half of the peanut butter mixture and spread evenly.
Pour all of the chocolate mixture on top and spread evenly.
Drizzle remaining peanut butter mixture evenly over the top. Swirl it in with a butter knife by gently moving the knife through the mixture in a serpentine pattern.
Refrigerate overnight or at least until set (about 4 hours).
Use at least a 3-quart saucepan for the evaporated milk and sugar mixture to avoid boiling over. A rolling boil is when the mixture is boiling and doesn't stop when stirred. The butter will melt more evenly and quickly if it is sliced into tablespoon portions.