Pit and quarter 2/3 of the sour cherries, leaving the rest whole.
Gently melt the dark chocolate and butter in a mixing bowl over a bain-marie. Then remove from steam and stir in the sugar well. Then stir in the flour briskly. Finally, stir in the eggs until the mixture is creamy. Gently stir in the quartered sour cherries.
Grease Rex jars well with lukewarm butter. Pour in the chocolate mixture until just above the center and bake in a preheated oven at 240 °C top/bottom heat for about 12-13 minutes.
Sprinkle the dark chocolate sour cherry cake with a little powdered sugar and garnish with the whole sour cherries.