For the cupcakes, preheat the oven to 350 degrees. Line a 12 cup cupcake pan with liners and set aside.
Melt the butter and chocolate together in the microwave in 30 second increments, stirring between each. Stir until combined and set aside to slightly cool.
In a medium bowl, whisk the cocoa powder, flour, baking soda, baking powder and salt until combined. Set aside. In a large bowl, whisk together the eggs, sugar, brown sugar and vanilla until well combined. Add the cooled butter and chocolate mixture and mix until smooth. Add half of the flour mixture then half of the buttermilk. Repeat until both are gone and whisk until just combined. Be careful not to over mix. The batter will be very thick.
Fill the cupcakes with one tablespoon of batter. Break the BUTTERFINGER® Cup Eggs 6pk in half and place a half in each cupcake tin. Place another tablespoon of batter on top of each and smooth until the BUTTERFINGER® is covered. Bake for 18 minutes or until tops of cupcakes are firm. Allow to cool completely.
While the cupcakes are baking, place 3/4 cup of BUTTERFINGER® Minis in a microwave safe bowl and heat on high for 20 seconds. Repeat until they're melted. Using a hand or stand mixer, beat the butter and peanut butter on medium speed until creamy and whipped, about two minutes. Add the vanilla and melted BUTTERFINGER® minis and beat until smooth. Sift in the powdered sugar and add to the butter mixture a cup at a time. Beat on low after each addition.
Frost cooled cupcakes with buttercream frosting using a piping bag or plastic freezer bag with the tip cut off. Sprinkle with the remaining BUTTERFINGER® Minis and place a BUTTERFINGER® NestEgg on the side.