Preheat your oven to 350°F (175°C). Grease a 6-cavity donut pan generously with nonstick spray or butter, making sure to coat the center posts well.
In a large mixing bowl, whisk together the flour, 1/3 cup sugar, baking powder, baking soda, 1/2 teaspoon cinnamon, and salt until evenly combined with no lumps.
In a separate medium bowl, whisk the egg until smooth. Add the buttermilk, 2 tablespoons melted butter, and vanilla extract, whisking until fully blended.
Pour the wet ingredients into the dry ingredients and gently fold with a spatula just until no dry flour remains visible. The batter will be thick but pourable — do not overmix or the donuts will be dense.
Transfer the batter to a piping bag or large zip-top bag with one corner snipped off. Pipe batter into each donut cavity until two-thirds full, leaving space for rising.
Bake for 10-12 minutes, until the tops spring back when gently pressed and a toothpick inserted comes out with just a few moist crumbs. Do not overbake.
Let donuts cool in the pan for 3 minutes, then carefully turn out onto a wire rack. While still warm, brush each donut all over with the 3 tablespoons melted butter.
In a shallow bowl, mix the 1/2 cup sugar with 1 tablespoon cinnamon. Roll each buttered donut in the cinnamon sugar mixture until fully coated on all sides.
Serve warm or at room temperature. These are best eaten the same day but will stay moist in an airtight container for 2-3 days.
