On medium speed, beat butter until light and fluffy.
Add Marshmallow Fluff; on low speed, beat until combined, about 30 seconds.
Gradually beat in confectioners’ sugar, then vanilla extract until blended, about one minute. Increase speed to medium and beat until fluffy, about 30 seconds.
Chop one Snickers candy bar. Transfer 2 ⅓ cups frosting to separate bowl and stir in chopped candy; reserve other for decoration.
In microwave-safe bowl, microwave chocolate chips in 15-second intervals until melted, stirring. Cool. Gently fold chocolate into remaining frosting.
Place one cake layer on serving plate; if necessary, trim top to level. Spread cake top with Snickers candy frosting
Top with remaining cake layer.
Spread top with chocolate frosting.
In microwave-safe bowl, microwave dulce de leche in 15 second intervals until pourable, about 45 seconds. Stirring at every interval.
Pour dulce de leche over cake, spreading with spatula.
Pour dulce de leche over cake, spreading with spatula. Slice remaining Snickers bar. Sprinkle candy and peanuts over cake.
Cut, serve and enjoy.