Preheat the oven to 360 degrees Fahrenheit/ 180 degrees Celsius. Line the molds of a muffin tin with paper cases.
Pit the cherries. Weigh the needed amount (250 g/ 8.8 oz/ 1 1/8 cup) after pitting. Let drain in a sieve while you prepare the batter.
Whisk the soft butter, sugar, vanilla extract, eggs, and milk in a bowl until combined.
In another bowl, mix cornstarch, flour, baking powder, and cocoa powder.
Batter: Gently start folding the dry ingredients into the bowl with the wet ingredients using a spatula. Once you have mixed the dry ingredients about halfway, add about 2/3 of the cherries. Mix again but take care not to over-mix, a few streaks of flour in the batter are OK.
Spoon the batter into the muffin tin. Place the remaining cherries on the muffins and sprinkle with the extra sugar.
Bake for about 25 minutes, checking after about 22 minutes already. A toothpick inserted in the middle of one muffin should come out clean.
Rest in the pan for about 5 minutes, then transfer to a wire rack to cool completely.