Using a small bowl, combine sugar and cinnamon; set bowl aside.

Using a countertop mixer, add 4 Tbsp of softened butter, sugar and salt; mix on medium speed until light and fluffy (~2-3 minutes). (Note: The remaining butter will be used after the dough has risen.)

Add the egg and mix until just combined.

Add the milk, yeast and flour; mix for 1-2 minutes on low speed, or until a smooth dough forms.

Increase speed to medium and mix for 3-4 more minutes.

Add ⅓ cup of sprinkles and mix on low speed for 1 minute, or until sprinkles have mixed into dough.

Cover dough and place in a warm location (85°F). Let rise for 1½ hours.

Once risen, transfer dough onto a lightly-floured countertop. Gently press dough into a 9”x12” rectangle.

Spread the remaining 4 Tbsp of softened butter evenly across the dough. Sprinkle cinnamon filling (from above) and remaining ⅓ cup of sprinkles evenly on top.

Beginning with a long edge, roll dough into a tight cylinder. Pinch seam closed. (Tip: Take care to keep ends of the cylinder even.)

Using a sharp knife, slice the cylinder into 8 pieces.

Grease the bottom of an 8” cast iron skillet with butter. Place the rolls cut-side down into the skillet.

Cover skillet and refrigerate overnight. (Note: If you want to bake these rolls the same day, you can simply cover the dough and let it rise for 60 minutes at room temperature. Then skip the next step and proceed with baking.)

The next morning, place skillet in a warm location until dough has filled bottom of the pan and increased in size (~90 minutes). (Note: Depending on how warm the spot is, this step may take a bit more or a bit less time.)

Bake at 325°F for 32-24 minutes, or until tops of rolls are golden brown.

Let buns cool in pan for 15 minutes.

While buns are cooling, make the glaze by whisking together the powdered sugar, milk and vanilla. Once buns have cooled slightly, spread the glaze on top and sprinkle with additional sprinkles.