Preheat your oven to 350°F (175°C) and grease a donut pan (or use a silicone one).
In a mixing bowl, combine the flour, sugar, baking powder, baking soda, salt, and ground cinnamon and whisk together until well combined.
In a separate bowl, whisk together the buttermilk, egg, melted butter, vegetable oil, and vanilla extract until smooth.
Gently fold the wet ingredients into the dry mix, stirring until just combined, being careful not to over-mix.
Transfer the batter into the donut pan using a piping bag or by spooning the batter into each cavity, filling them about 3/4 full.
Bake in the preheated oven for 10-12 minutes, or until a toothpick inserted into the center comes out clean.
Let the donuts cool for a few minutes before removing them from the pan.
Mix sugar and cinnamon in a small bowl and roll each warm donut in the mixture until well coated.
Serve immediately and let the smiles follow!