Preheat the oven to 350°F (175°C). Grease a 6–12 cup donut pan lightly with oil or butter.

In a large bowl, whisk together 2 cups flour, 1 cup sugar, 1 1/2 tsp baking powder, and 1 tsp baking soda until evenly mixed.

In a separate bowl, combine 1 1/4 cups milk, 6 tbsp melted butter, and 1 tsp vanilla extract.

Pour the wet ingredients into the dry. Stir gently with a spatula until just combined. The batter should be thick but spoonable — don’t overmix.

Transfer batter to a piping bag or a zip-top bag with the corner snipped. Pipe or spoon batter into the donut pan, filling each cavity about two-thirds full.

Bake for 9–12 minutes, or until the tops spring back when lightly touched and a toothpick comes out clean.

Let donuts cool in the pan for 2 minutes, then transfer to a wire rack.

In a shallow bowl, mix 1/4 cup sugar with 1 tsp cinnamon. Brush each warm donut with 2 tbsp melted butter and immediately toss or dip into the cinnamon sugar so it adheres.

Let the coated donuts cool slightly before serving so the sugar sets.