In a heavy-bottomed saucepan, pour in the whole milk and set it over medium heat. Heat until it reaches 195°F, stirring constantly with a whisk to prevent scalding.

While the milk warms, combine the all-purpose flour, granulated sugar, and kosher salt in a separate bowl. Whisk until blended.

In a small saucepan, melt the salted butter over low heat until bubbly and fragrant, then set aside.

Gradually stir the flour mixture into the melted butter, mixing until a smooth paste forms.

Slowly add the butter-flour paste to the heated milk, whisking continuously until thickened and boiling.

Reduce heat to low and let the Rømmegrøt simmer for about 5 minutes, stirring frequently.

Ladle Rømmegrøt into warm bowls, drizzle with melted butter, and sprinkle with cinnamon sugar to taste.