Separate the egg yolks into one bowl.
Place the egg whites in a separate bowl.
Beat the egg whites with a mixer until they form stiff peaks.
Add the sugar and beat again for a short time until you get a glossy, firm meringue.
Add the oil to the yolks and whisk with a hand whisk until the mixture is smooth.
Pour the yolk mixture over the whipped egg whites.
Mix briefly with the mixer on the lowest speed, just for a few seconds, until combined.
In a separate bowl, whisk together the baking powder and flour.
Gradually add the flour mixture to the batter, gently folding with a spoon or spatula.
The batter should be thick and fluffy at this point.
Rinse the sour cherries and remove the pits if needed.
Add about one‑third of the cherries to the batter and fold them in gently.
Pour the batter into a baking pan lined with parchment paper, then spread it into an even layer.
Arrange the remaining cherries over the top of the batter. Bake in a preheated oven at 350°F (175°C) for 20–25 minutes, or longer if needed, depending on your oven.
When the sponge cake with sour cherries is done, let it cool, then slice it and dust with powdered sugar before serving.