In a large mixing bowl, combine the flour, baking powder, and salt. In another bowl, whisk together the milk, buttermilk, melted butter, and 1 cup of granulated sugar until well-blended. Gradually add the wet ingredients to the dry ingredients, stirring until just combined. Be careful not to overmix; a few lumps are perfectly fine.
In a deep pan or fryer, heat oil over medium-high heat. You want enough oil to submerge the bonuts about halfway. Test the oil temperature by dropping a small amount of the dough; it should sizzle and rise to the surface.
Once the oil is hot, use a spoon or cookie scoop to drop small portions of the dough into the oil. Fry in batches to avoid overcrowding. Cook each bonut for about 2-3 minutes on each side or until they are golden brown.
As soon as the bonuts are done frying, remove them with a slotted spoon and let them drain on a cooling rack. In a separate bowl, mix together the granulated sugar and cinnamon. While the bonuts are still warm, roll them in the cinnamon sugar mixture until fully coated.
To prepare the glaze, combine the powdered sugar, milk, and vanilla in a bowl. Whisk until smooth and drizzle over the warm bonuts. Allow the glaze to set for a few minutes before serving.