The PB cake is soft and creamy.
The honeycomb frosting will hurt your teeth.
The Butterfinger bars are an absurd textural necessity.
This is not a cake for the faint-hearted.

Peanut butter cake based on Leelabean Bakes.
Honeycomb frosting from Raspberri Cupcakes.
Butterfinger topping from…some other planet. (Seriously those things are weird.)

Gather:
For the cake
2 cups cake flour
1 tsp baking powder
1 tsp baking soda
1/2 tsp salt

110 g butter (room temp.)
1/2 cup smooth peanut butter
1 cup brown sugar
1/2 cup caster sugar
3 eggs
1/2 cup buttermilk
1 tsp vanilla

For the frosting*
2 full-size Crunchie (or Violet Crumble) bars
250 g butter (room temp.)
4 cups icing mixture
2 tbsp golden syrup
2 tbsp honey

For the topping
2 Butterfinger bars

Then:

1. Begin with the cake.
   Preheat the oven to 175° C and prepare two springform cake tins (butter the base and sides, and line the bottom with baking paper).
2. If you need to prep your own cake flour do that. Then grab a medium-sized bowl and sift together the flour, baking soda and powder, and salt. Set aside.
3. In the bowl of your stand mixer (or a large bowl with hand beaters) mix the butter and peanut butter on medium speed, until there are no lumps of either.
4. Still mixing, add the sugar and mix for a few minutes until the granules have dissolved.
5. Slow down the beaters/mixer, and add the eggs one by one, fully incorporating each one before adding the next.
6. Mixing on the slowest speed, add the vanilla, then start alternately adding the buttermilk and dry ingredients bit by bit until it’s all in there. Stop when the mixture is smooth and incorporated — don’t overdo it.
7. Divide the batter between the two cake tins, smooth out the tops with a spatula, then bake — on the same shelf if possible, swapping sides halfway through — for about 30 minutes. Stick a bamboo skewer in the centre of each cake towards the end — if it comes out clean the cakes are done.
8. Remove the cakes from the oven and leave them — in the tins, on a cooling rack — until they’re at room temperature or cooler.
   
   …
9. Now for the frosting.
   First, smash up the Crunchie/Violet Crumble bars into very small pieces — mostly dust, with a few rock-salt–sized chunks.**
10. Sift the icing mixture and set aside.
11. Cut the butter into small cubes/pieces, and beat it on high speed with your stand mixer/in a large bowl with hand mixers until it’s pale and shiny.
12. Still mixing, but very slowly, add the icing mixture a tablespoon or so at a time. Start adding the golden syrup and honey as the mixture becomes thicker/drier.
13. Once you’ve got a sturdy but still spreadable consistency, add the crushed honeycomb pieces and mix until just combined.***
    …
14. To assemble:
    Turn the bottom cake upside down and spread a 5mm layer of frosting on the flat surface. Carefully sandwich the two cakes together, with the flat (bottom) surface of the top cake facing up (to avoid Butterfinger pieces tumbling off a sloping top).
15. Spread a layer of frosting over the entire cake, and make sure the top is flat.
16. Crush the Butterfinger bars by hand — they’re very easily crumbled — in an even layer all over the top of the cake.
17. Serve with a glass of milk, and don’t tell your dentist.
    
    (Since when are Butterfinger bars so orange?)

Notes
*This recipe makes more than enough frosting for a layer in between the two cakes and a thick layer all over the outside — by which time your teeth really will hurt. If you want to tone it down a little bit, you could probably halve this frosting recipe, and just spread a thin layer of peanut butter in the middle instead.
**I did this by hand, but you could put them in a bag and smash them with a rolling pin — or use a food processor, of course.
***At this point, if you’re planning to transport the cake or assemble it later, transfer the frosting to an airtight container and keep it in the fridge until an hour (or two) before you’re ready to assemble — it will become spreadable again at room temperature.