In a large bowl, combine softened butter, vegetable oil, sugar, egg yolk, vinegar, and salt. Mix until smooth and creamy.
Add mahlep and baking powder, then gradually stir in the flour. Use your hands to bring the dough together into a soft, smooth ball. It shouldn’t stick to your fingers.
If it feels too dry, add a teaspoon of oil. If it’s too sticky, sprinkle a little more flour. Cover the bowl and let the dough rest for about 10 to 15 minutes. This short pause makes shaping easier and keeps the cookies neat while baking.
Line a baking tray with parchment paper and preheat the oven to 180°C (350°F).
Pinch off small walnut-sized pieces of dough. Roll each one into a thin rope, about 10 to 12 cm long. Join the ends to form a ring and press gently so they stick together.
Lightly whisk the egg white and brush each ring with it, then sprinkle a generous amount of sesame seeds on top. I add some nigella seeds to add a bit of color, but it is entirely optional.
Place the rings on the tray, leaving a bit of space between them. Bake for 15-18 minutes, or until golden. The sesame seeds should look toasted, and the bottoms should be lightly browned.
Let them cool on the tray for a few minutes, then move to a rack to cool completely.