Preheat your oven to 325°F. Liberally prepare 2 mini Bundt pans with 5-cup or 6-cup capacity with the nonstick method of your choice.

In the bowl of your stand mixer fitted with the whisk attachment, beat the butter on high speed for 1 minute. Slowly add the granulated sugar. Cream together for an additional 5 minutes, until very pale yellow and fluffy. Add the eggs 1 at a time, combining well after each addition and scraping down the sides and bottom of the bowl as needed.

Turn your mixer down to its lowest speed and slowly add the flour in 2 batches. Add the salt and baking soda. Be careful not to overbeat. Add the sour cream, oil, and vanilla extract. Scrape down the sides and bottom of the bowl and mix the batter until just combined. Be careful not to overmix.

Evenly separate the batter into 2 medium bowls. Set 1 bowl aside.

Whisk the cocoa powder into the second bowl and set aside.

Place alternating tablespoons of both batters into each cavity of your prepared pans, until each cavity is ⅔ full. Start with the vanilla, adding a tablespoon to each cavity, then add the chocolate. Start again with the batters until each cavity is ⅔ full. Be careful not to overfill. Bake for 24 to 28 minutes, or until a toothpick inserted into the center of a cakelette comes out mostly clean but still moist.

Let the cakelettes cool in the pan for 10 minutes, then invert onto a serving plate. Let cool to room temperature. Lightly cover the cakelettes with foil or plastic wrap so they not dry out.

In your stand mixer, add butter and salt at medium low speed.

Next turn the mixer down to low and slowly add powdered sugar in small portions until all is added then turn the speed to medium high to combine. Mix until fluffy.

Separate half of buttercream into a medium bowl and stir in cocoa powder.

Add both buttercreams to separate piping bags then place both side by side into piping tip. Pipe frosting on each cakelette in the center hole and serve.