Heat the whole milk in a heavy-bottomed saucepan over medium heat until it reaches 195°F, making sure to stir gently to prevent burning.

Whisk together the all-purpose flour, granulated sugar, and kosher salt in a separate bowl until combined. Set aside.

Melt the salted butter in a small saucepan over low heat, stirring occasionally, and remove from heat once melted.

Stir the flour mixture into the melted butter to form a smooth paste, then gradually add this to the hot milk while whisking constantly.

Reduce heat to low and cook the mixture for an additional 5 minutes, stirring continuously until it thickens.

Spoon the warm Rømmegrøt into bowls, drizzle with melted butter, and sprinkle with cinnamon sugar.