Place a large roasting pan in the oven. Fill it with 2 inches of boiling water. Preheat oven to 350 degrees F.
Grease a 12 - 14 cup Bundt pan with butter.
To make the caramel: place a heavy sauce skillet on the stove over medium heat and melt 1 cup of sugar, swirling it around occasionally until it melts completely. Keep an eye on it so it doesn't burn.
Wearing oven mittens, immediately pour the caramel into the greased Bundt pan, moving the caramel around to coat the surface. Let the sugar cool down while you prepare the cake batter.
To make the cake: In a large bowl beat butter and sugar well, until fluffy. You can use your mixer for this or do it manually. Beat in the egg until it has fully incorporated.
In a separate bowl, sift the dry ingredients: flour, baking powder, baking soda and cocoa powder. Sifting the ingredients prevents clumps.
Alternate adding the flour mixture and the buttermilk into the sugar and butter mixture, starting with the flour. Incorporate flour completely, then add buttermilk, then flour again and so on. Beat until all the ingredients are incorporated.
Spoon all the cake batter into the Bundt pan, smoothing out the top.
To make the flan: Mix all the flan ingredients in the blender on high for 1 minute and pour on top of the cake batter. Cover the Bundt pan with foil and place it into the roasting pan with water in the oven. The water shall come up to almost the lowest half of the pan.
Bake for 1 hour and 40 minutes. Turn the pan halfways when the baking hits the 50 minute mark, to ensure an even cooking.
Carefully remove the Bundt pan from the oven and let it cool down at room temperature for 2 hours.
To unmold the Peanut Butter Chocoflan, run a thin-blade knife around the edge of the Bundt pan. Place an inverted rim platter on top of it and tightly grasping both platter and pan jiggle and flip over. Scrape any caramel left in the pan over the cake. Refrigerate, ideally for 24 hours.
To make the garnish, beat the heavy whipping cream until soft peaks form. Garnish cake with the whipped cream, the peanut butter cups, the sprinkle mix and the chocolate candies, and serve. If you want a simpler garnish, just add toasted, chopped peanuts on top of the cake. Keep refrigerated.