In a large, heavy-bottomed saucepan, pour in the whole milk and place it over medium heat. Stir frequently and monitor the temperature until it reaches 195°F. Be careful not to let it boil.

In a small bowl, whisk together all-purpose flour, granulated sugar, and kosher salt until well combined. Set aside.

In a separate small saucepan, add the salted butter and melt it over low heat. Keep stirring until fully melted.

Once the milk reaches the desired temperature, stir the flour mixture into the melted butter, creating a thick paste. Whisk continuously.

Slowly drizzle the thick paste into the hot milk while whisking vigorously. Continue stirring until the mixture begins to boil and thicken, about 3–5 minutes.

Once thickened, remove from heat and spoon the warm porridge into individual bowls. Drizzle each serving with melted butter and sprinkle with cinnamon-sugar.