In a large mixing bowl, combine softened butter, liquid oil, egg, water, salt, sugar, baking powder, vinegar, and mahlep. Stir or whisk until the mixture becomes smooth and unified. The aroma of mahlep will give the dough a warm, nutty note.
Begin adding the flour gradually, mixing after each addition. Knead the mixture by hand or with a stand mixer until you achieve a soft, pliable dough that’s not sticky and holds its shape well.
Pinch off small pieces of dough, roughly the size of a walnut. Roll each one into a thin rope, about the thickness of your finger, either between your palms or on a clean surface. Twist or leave straight, then bring the ends together to form a neat ring.
To prepare sesame-coated simit, whisk together the water and grape molasses in a shallow bowl. Dip each dough ring into the molasses mixture, ensuring it's fully coated. Then, press both sides into a plate filled with sesame seeds, allowing a generous coating to cling. Arrange the coated rings on a parchment-lined baking tray.
For the version with nigella seeds, whisk together the egg yolk and a spoonful of molasses. Place the plain dough rings on the tray, brush the tops generously with the egg yolk mixture, and sprinkle over the nigella seeds to your preference.
Bake in a preheated oven at 180°C (350°F) for about 20 minutes or until golden brown and slightly crisp. Monitor closely, as oven temperatures can vary. The simit should be deeply golden with a fragrant, toasty crust when done. Let cool slightly before serving.