Drain cherries, preheat oven:

Thoroughly drain the sour cherries using a sieve. Line a muffin pan with paper liners. Set the oven to preheat to 350°F (175°C).

Make batter, layer and bake:

Using an electric mixer, cream the soft butter or margarine in a bowl until smooth. Slowly incorporate the sugar, vanilla sugar, and salt, beating until the mixture becomes thick and creamy. Beat in the eggs one by one, each for about 30 seconds on high speed. Combine the flour with the baking powder and fold into the batter on medium speed in two additions, mixing only until combined. Portion 2/3 of the batter into the paper liners. Mix the cocoa powder and milk into the remaining batter. Spoon this dark batter over the light batter. Scatter half of the cherries on top and gently press them in. Bake the muffin pan on the oven rack for 25 minutes. Take the muffins out of the pan and allow them to cool on a wire rack.

Prepare buttercream, pipe rosettes:

To prepare the buttercream: Beat the soft butter and powdered sugar in a bowl with an electric mixer until smooth. In another bowl, whisk together the heavy cream and pudding cream powder for 1 minute. Gradually add spoonfuls of the cream mixture to the butter, beating thoroughly after each addition. Make sure all ingredients are at room temperature to avoid curdling. Fill a piping bag fitted with a star tip (13 mm diameter) with the cream and pipe large rosettes onto the cooled muffins. Place the remaining cherries in the center of each rosette. Chill the muffins in the refrigerator for about 30 minutes.

Melt chocolate, drizzle on muffins:

For the chocolate drizzle: Coarsely chop the dark chocolate coating and place it in a small freezer bag; seal the bag tightly. Melt the chocolate in a water bath over low heat. Allow the chocolate to cool a bit. Dry the bag, snip off a tiny corner, and drizzle the chocolate over the muffins.