In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a stand mixer fitted with the paddle attachment, beat butter and sugars on med-high until light and fluffy. Approx 2-3mins.
Add peanut butter and vanilla, beat until thoroughly combined.
Reduce speed and add eggs one at a time, thoroughly incorporating after each addition.
Reduce mixer to low. Add flour mixture in 3 portions, alternating with 2 portions of milk. Mix until just combined.
Pour half of the batter into a medium bowl, set aside.*
Add 3 Tbsp cocoa powder to remaining batter and return to stand mixer. Beat until just combined.
Prepare 2 separate pastry bags and fill one with chocolate batter and the other with peanut butter.
Pipe the batter, alternating the colors, into the cracks of the Heritage bundt pan, being careful to avoid spilling over into neighbouring cracks. **
Once cracks are sufficiently filled, add remaining batter in alternating layers and use a kitchen skewer to marble it. Spread the batter evenly.
Bake for 45-50mins or until a skewer comes out clean.
Remove from oven and transfer bundt pan to cooling rack for 10-15mins. Rap the bundt pan against the counter to loosen it then turn cake out onto cooling rack to cool completely.
Line a baking sheet with parchment or a silpat.
Place sugar and water into a small pot over med-high heat. Stir only to combine.
Wash down the sides of the pot with some water and a pastry brush to prevent sugar from crystallizing. Do this from time to time, as needed, while the sugar is cooking.
Continue to cook sugar until the caramel reaches a medium amber color. Do not stir at any point.
Add in peanuts and salt and stir for 1 minute to coat.
Pour peanuts onto baking sheet and spread evenly. Allow to cool completely, then roughly chop.
Place marshmallow fluff, and icing sugar into a medium bowl, stir well to combine. Add 2 Tbsp of cream and stir until glossy and smooth. Add more cream as needed until desired consistency is reached.
Using a small measuring cup, pour glaze carefully into the cracks of the cooled bundt cake.
Sprinkle top with chopped candied peanuts.
*I use a kitchen scale to weigh the batter to ensure I get exactly half, but a normal person would probably just eyeball it. **I used a kitchen skewer for the finer points of the cracks. It's a process.Bundt cake recipe adapted from Bake or Break.