Preheat oven to 350°F (175°C). Grease and flour one 8-inch round cake pan. Line the bottom with parchment paper.
In a medium bowl, whisk flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, whisk melted butter, eggs, and vanilla. Pour wet ingredients into dry, mix until just combined.
Pour batter into the prepared pan.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs. Do not overbake.
Let cool completely in the pan on a wire rack.

While the base cools, grease and flour two more 8-inch round cake pans. Line the bottoms with parchment paper.
In a large bowl, whisk together flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt.
Add buttermilk, vegetable oil, eggs, and vanilla extract. Beat with an electric mixer on medium speed for 2 minutes.
Carefully stir in the hot brewed coffee (batter will be thin).
Divide batter evenly between the two prepared cake pans.
Bake for 30-35 minutes, or until a wooden skewer inserted comes out clean.
Let cool in pans for 10-15 minutes, then invert onto wire racks to cool completely.

In a large bowl, using an electric mixer, beat the softened butter until creamy.
Add the creamy peanut butter and beat until smooth.
Gradually add the sifted powdered sugar, alternating with milk/cream, until desired consistency is reached. Beat until light and fluffy.
Beat in vanilla extract and salt.

In a medium saucepan, heat heavy cream over medium heat until it just begins to simmer (do not boil).
Remove from heat and pour over finely chopped chocolate in a heatproof bowl. Let sit 5 minutes, then whisk until smooth and glossy.
Let cool slightly at room temperature until thick but still spreadable.

Level Cakes: Ensure all cake layers (including the base) are level.
Chocolate Base: Place the chocolate base layer on your serving plate or cake stand.
First Peanut Butter Layer: Spread a generous layer (about 1.5 cups) of peanut butter frosting over the chocolate base.
First Chocolate Fudge Layer: Carefully pour/spread about ⅔ cup of the slightly cooled chocolate fudge/ganache over the peanut butter frosting.
First Chocolate Cake Layer: Carefully place one chocolate cake layer on top.
Second Peanut Butter Layer & Candy: Spread another generous layer (about 1.5 cups) of peanut butter frosting over this cake layer. Sprinkle about half (4-5 bars worth) of the chopped candy bar pieces onto this frosting, pressing them gently.
Second Chocolate Fudge Layer: Pour/spread another ⅔ cup of chocolate fudge/ganache over the candy/peanut butter layer.
Second Chocolate Cake Layer: Place the final chocolate cake layer on top.
Crumb Coat: Apply a thin layer of peanut butter frosting all over the top and sides of the cake. Refrigerate for 15-20 minutes to set.
Final Coat: Apply a thicker, even layer of peanut butter frosting around the sides of the cake, leaving the top mostly covered with the previous fudge layer. You can create decorative swirls or a smooth finish on the sides.
Chill: Refrigerate the cake for at least 1-2 hours to allow the frosting and layers to set firmly.

Top Ganache: If the chocolate ganache for the top has set too much, warm it gently until it’s pourable again. Pour the remaining ganache (about ⅔ cup) over the top of the cake, spreading it smoothly to the edges and allowing some to drip down the sides of the cake.
Piped Frosting: Transfer some remaining peanut butter frosting to a piping bag fitted with a star tip. Pipe decorative swirls or dollops around the top edge of the cake.
Drizzles: Drizzle extra chocolate ganache and/or caramel sauce over the top of the cake, including the piped frosting.
Candy Garnish: Arrange the remaining chopped candy bar pieces and optional chopped peanuts on top of the cake, especially on the piped frosting and open areas.
Final Chill: Refrigerate the decorated cake for at least 1-2 hours before slicing to allow all elements to set.