sift the flour and cocoa into a bowl. Add the egg, egg yolk, and oil, then gradually beat in the milk until smooth. Let stand for 15 minutes.
For the filling, put the cherries into a pan. Mix the cornstarch to a smooth paste with a little of the water then add to the pan with the remaining water and the sugar. Cook over medium heat for 45 minutes, stirring, until the sauce has thickened and the cherries are slightly softened.
Lightly beat the mascarpone cheese, custard, and vanilla extract together in a bowl until smooth.
Heat a little oil in a small skillet, then pour the excess into a cup. Add 2 to 3 spoonfuls of the crepe batter to the skillet, tilting to swirl into a thin lacy pancake. Cook for 1 to 2 minutes until browned, then turn over with a spatula and cook the second side. Slide the pancake onto a plate and repeat with the remaining batter to make eight crepes.
Divide the crepes among serving plate. Spoon the mascarpone mixture over the center of each pancake, top with the cherry filling, then fold each pancake into quarters. Dust lightly with sifted powdered sugar and serve at once. Serves four.