Preheat oven to 350ºF. Generously butter the bottom and sides of an 8-inch cast iron (or other oven-proof skillet), then dust with flour.

In a medium bowl, whisk the sugar, melted butter, egg and vanilla until blended.

Add the flour, salt, milk and sour cream; whisk until just combined and there are no visible traces of flour (it’s okay if there’s a few lumps).

Stir in the sprinkles.

Pour into the prepared skillet. Bake on center rack until cake gently pulls away from side of the skillet, center is firm when gently tapped with your fingers and cake is deep golden around the edges, 35 to 45 minutes (start checking at 35 minutes).

Set skillet on a wire rack and let cake cool completely before serving.