Make the cakes:

1. Grease and flour the bundt pan cavities thoroughly, or spray with baking spray.
2. Sift the flour, sugars, cocoa, baking soda, baking powder, and salt into the bowl of an electric mixer fitted with a paddle attachment. Mix on low speed until combined.
3. In a separate bowl, whisk together the buttermilk, oil, eggs, and vanilla.
4. With the mixer on low speed, slowly add the wet ingredients to the dry until fully combined. Scrape the bowl. With the mixer still on low, add the hot coffee and stir until just combined — batter will be thin.
5. Pour batter into the prepared mini bundt pan cavities, filling each about 3/4 full. Bake at 325°F (165°C) for 25–30 minutes, until a cake tester comes out clean. Cool in the pans for 30 minutes, then turn out onto a cooling rack and cool completely.

Make the buttercream:

1. Beat the butter with an electric mixer until pale and fluffy, about 3 minutes.
2. Add the sifted powdered sugar in two additions, mixing on low between each, then increase to medium.
3. Add the heavy cream, vanilla, and a pinch of salt. Beat on medium-high for 2–3 minutes until light and smooth. Add a splash more cream if needed to reach a spreadable consistency.

Assemble:

1. Slice each cooled bundt cake in half horizontally. Spread or pipe a generous layer of vanilla buttercream on the cut side of the bottom half. Press some cereal into the buttercream.
2. Melt the milk chocolate chips and drizzle over the top of the assembled cakes. Pile on more cereal for garnish. Serve immediately or refrigerate until ready to serve.