In a small bowl, combine granulated sugar and ground cinnamon, mixing thoroughly until well blended. Set aside for coating.

Carefully melt unsalted butter in a microwave-safe bowl or saucepan over low heat, ensuring it liquefies without overheating.

In a large mixing bowl, pour in the plain toasted oat cereal. Drizzle the melted butter evenly over the cereal and gently stir to coat.

While stirring, sprinkle the prepared cinnamon-sugar mixture over the butter-coated cereal gradually, using a folding technique.

Spread the coated cereal mixture onto a parchment-lined baking sheet in an even layer and allow it to cool for about 10-15 minutes.