PREPARE THE EQUIPMENT: Preheat the oven to 350º F. Lay your 6" cake pan(s) on parchment paper, trace around them with a pencil until you have 3 circles, and cut them out with kitchen scissors. Grease the bottom and sides of each pan, lay the parchment circles inside, and grease the top of the parchment. Set the pans aside. Use a kitchen scale to take the tare weight of the bowl in which you'll make your cake batter.
MAKE THE CAKES: Whisk together the flour, cocoa powder, baking powder, baking soda, and salt in the bowl you just weighed. In a separate bowl, combine the eggs, both kinds of sugar, oil, and vanilla. Pour half of the wet ingredients into the dry, add half the buttermilk, and stir to combine. Add the rest of the wet ingredients and buttermilk and stir until just incorporated. Don't overmix.
BAKE: Weigh the bowl of cake batter and subtract the weight of the bowl. Divide the remainder by 3; this is the amount of batter you'll put in each pan. Divide the batter evenly between the pans and bake for 19-21 minutes, or until a toothpick inserted in the center comes out clean.
COOL THE CAKES: As soon as the cakes come out of the oven, turn them out onto a cooling rack. Don't peel off the parchment until the cakes have cooled to room temperature. At this point, transfer the cakes to the fridge to cool completely, about 1-2 hours. Meanwhile, make the frosting.
BUTTERCREAM FROSTING: In two separate bowls, combine the ingredients for each buttercream using a whisk or electric hand mixer. Beat until no lumps remain. For the salted caramel peanut butter buttercream, the amount of sugar added will depend on the peanut butter and caramel sauce you use. Start with one cup and continue to add by the spoonful until you have the consistency you want. Prepare two piping bags or cut a very small tip off one corner of two Ziploc bags. Fit a medium-sized round tip on one bag and add the chocolate buttercream. Fit a star tip on the other bag and add the peanut butter and salted caramel buttercream. Keep the frosting at room temperature until ready to use.
ASSEMBLE THE CAKE: Add one layer of cake to the surface on which you'll construct your cake (a plate, cake stand, or cutting board would work). Pipe a tight spiral of peanut butter frosting on top. Add the second cake layer and repeat with more frosting. Add the final layer and make sure the layers are lined up as evenly as possible. Don't frost the top of the cake yet. Pipe extra frosting to fill in any gaps in the layers – you want the cake to be as cylindrical as possible. Chill the cake again for about 20 minutes.
FROST THE CAKE: Use an offset spatula or butter knife to spread a generous layer of chocolate buttercream over the top and sides of the cake. Prepare a cup of very hot water and rinse off the knife after each stroke; this helps get the frosting completely smooth. Work quickly so the hot water doesn't melt the buttercream. Chill for another 10-20 minutes, or until set.
FINISHING TOUCHES: Wipe down the surface around the cake. Pipe rings of stars around the bottom and top edges of the cake with the peanut butter frosting. Decorate with additional piping, fruit, nuts, or anything else you wish. Chill completely and serve cold.