Preheat your oven to 180°C (350°F).
Combine 1/2 tablespoon vinegar with 1/2 cup milk and let it sit for about 5 minutes until it curdles, forming buttermilk.
Brush a donut pan with 1 tablespoon of melted butter to prevent sticking.
In a large mixing bowl, add 1 cup flour, 3/4 cup sugar, 1 teaspoon baking powder, and 1/2 teaspoon salt.
Add 2 large eggs, 1/4 cup sunflower oil, 1 teaspoon vanilla extract, and the prepared buttermilk. Mix until the batter is smooth and well-combined.
Pour the batter into the greased donut pan, filling each mold up to 3/4 full.
Place the pan in the preheated oven and bake for 12-15 minutes, or until the donuts are light golden brown.
Allow the donuts to rest in the pan for 5 minutes.
Run a knife around the edges of each donut to gently unmold them.
Press each warm donut into a plate of sugar to coat the surface.
Enjoy these delicious donuts with a freshly brewed cup of coffee!