In a heavy-bottomed saucepan, pour in the whole milk and place it over medium heat. Stir continuously until heated to around 195°F, just before boiling.

While the milk warms, whisk together the all-purpose flour, granulated sugar, and kosher salt in a mixing bowl until smooth and free of lumps.

In a separate saucepan, melt the salted butter over low heat until fully melted, then remove from heat.

Mix the flour-sugar-salt blend into the melted butter to create a thick paste.

Gradually whisk the flour paste into the heated milk, stirring vigorously. Bring to a gentle boil, then reduce heat and simmer for about 5 minutes, stirring continuously.

Spoon the thickened Rømmegrøt into bowls. Drizzle with melted butter and sprinkle with cinnamon-sugar before serving.