Set the temperature of the oven to 350°F (175°C). Use a cooking spray that is non-stick to grease a donut pan.
Mix the flour, sugar, salt, baking powder, baking soda, and ground cinnamon in a big bowl.
Combine the buttermilk, egg, melted butter, and vanilla extract in a separate basin.
After adding the liquid components, mix the dry ingredients only until they are barely blended. Avoid overmixing.
Pour the batter into a corner-snipped resealable plastic bag or a piping bag.
Fill each cavity in the doughnut pan evenly with the batter, about two-thirds full.
Until a toothpick inserted into the donuts comes out clean, bake for 12 to 15 minutes.
After removing the donuts from the oven, give them some time to cool in the pan.
For the coating, mix the ground cinnamon and granulated sugar in a small basin. Each doughnut should be covered completely with cinnamon sugar after being dipped in melted butter.
Place the coated donuts on a wire rack to cool completely.