Preheat your oven to 375°F (190°C). Spray a standard 6-cavity donut pan generously with nonstick cooking spray, making sure to coat all the ridges.
In a large mixing bowl, whisk together the flour, 1/2 cup sugar, baking powder, 1/2 teaspoon cinnamon, and salt until evenly combined with no lumps.
In a separate medium bowl, whisk together the milk, egg, 3 tablespoons melted butter, and vanilla extract until smooth and fully blended.
Pour the wet ingredients into the dry ingredients and gently fold with a rubber spatula just until the flour disappears. The batter should be thick and slightly lumpy — do not overmix.
Transfer the batter to a piping bag or a large zip-top bag with one corner snipped off. Pipe the batter into each donut cavity, filling about two-thirds full to allow room for rising.
Bake for 10-12 minutes, until the donuts spring back when lightly touched and a toothpick inserted comes out clean. They should be golden around the edges.
Let the donuts cool in the pan for 3 minutes, then carefully turn them out onto a wire rack.
While the donuts are still warm, brush each one generously with the remaining 6 tablespoons melted butter, coating all surfaces.
In a shallow bowl, combine 3/4 cup sugar with 2 tablespoons cinnamon. Roll each butter-coated donut in the mixture, pressing gently so the coating sticks evenly on all sides.
Serve immediately while warm, or let cool completely before storing. The coating sets best within 5 minutes of applying.
