Add the wet ingredients in a large mixing bowl: melted butter, granulated sugar, eggs, buttermilk, and vanilla extract. Stir with a wooden spoon or whisk.
Add the dry ingredients to the bowl: cake flour, baking powder and salt. Stir until combined.
Use a piping bag to pour the batter perfectly into each mini donut shape on the iron. Close the iron and cook to the manufacturer's instructions.
Remove and serve immediately with desired toppings or cover until all the donuts are baked and serve all together.