For the sponge layers.
Mix the eggs with sugar and sour cream. In another bowl, combine the flour with baking powder and baking soda.
Add to the wet mixture and stir well.
Pour half of the batter into a 16 cm pan lined with foil.
Add cocoa to the remaining batter, mix well, and pour into another 16 cm pan.
Bake both sponges for 20–25 minutes at 170 °C (340 °F). Remove and let cool completely.
For the sour cream filling.
Bloom the gelatin in water for 15–20 minutes.
In a saucepan, whisk together sugar and cornstarch, pour in the milk, stir again, and bring to a boil. Cook for 1 minute.
Transfer to a bowl, cover with plastic wrap, and refrigerate for 1 hour.
Whip the sour cream with the gelatin melted in the microwave, then add the chilled custard and whip for 4–5 minutes. Transfer the cream to a piping bag if desired.
For the syrup-soaked plums.
Pour liqueur over the plums and let them soak for at least 1 hour.
Assembling the cake.
Trim the tops of the cooled sponges and cut each into two layers.
Place one cocoa sponge layer into an 18 cm pan, cover the sides and top with cream, sprinkle with diced plums. Add a sour cream sponge on top, then repeat with cream, plums, and remaining sponge layers.
Refrigerate for at least 3 hours.
For the glaze.
Melt chocolate with butter in the microwave. Whip sour cream with sugar, add the melted chocolate mixture, and blend with an immersion blender for 2–3 minutes.
Decoration.
Remove the cake ring, pour the glaze over the cake. Decorate the sides with crumbled cocoa sponge and add extra plums if desired.
