Preheat the oven to 350 degrees F (175 C). Line a cupcake pan with cupcake liners.

In a medium bowl, combine the all-purpose flour, cocoa powder, baking soda, baking powder, and salt. Whisk together.

In the bowl of a stand mixer, add the sugar and eggs and beat on medium-high speed until light and smooth.

Add the vegetable oil, buttermilk, and vanilla extract. Mix all the ingredients together until well combined.

Turn the mixer to low speed and slowly add in the dry ingredients until just combined. Do not overmix.

Spoon about 0.25 cup of batter into each cupcake liner. Place the cupcake pan into the preheated oven and bake for 15 to 20 minutes, or until a toothpick inserted into the center comes out clean.

Once the cupcakes are done baking, remove them from the pan and place them onto a wire cooling rack. Cool completely before frosting.

While the cupcakes cool, prepare the ganache filling.

Start by chopping the bittersweet or semisweet chocolate into small pieces and place them in a medium, heatproof bowl.

Add the heavy cream to a small saucepan and heat over medium-low heat until it begins to simmer. Quickly pour the hot cream over the finely chopped chocolate. Let the chocolate sit for about 5 minutes to melt.

Mix the cream and chocolate together until it becomes a smooth and shiny ganache. Let the ganache cool for about 15 minutes, stirring occasionally.

Once cupcakes are completely cool, use a cupcake corer or a small knife to carefully core out the middle of the top of each cupcake.

Spoon or pipe the cooled ganache into each cored cupcake.

For the buttercream frosting, in the bowl of a stand mixer, add the softened unsalted butter and creamy peanut butter. Mix until light and creamy.

With the mixer on low speed, slowly add the powdered sugar. Once incorporated, add the milk a little at a time until the frosting reaches a smooth, spreadable consistency.

After the frosting is done, use a large cookie scoop or a piping bag to add the frosting on top of each filled cupcake.

Generously sprinkle the crushed Butterfinger candy bars over the frosting.

The Butterfinger bars can be served immediately or stored in an airtight container for later.