In a small bowl, stir together warm milk, active dry yeast, and 1 teaspoon granulated sugar. Let sit until foamy, about 5-10 minutes.

In a large bowl, combine the remaining granulated sugar, room-temperature eggs, softened unsalted butter, and salt. Mix until combined.

Add the yeast mixture to the egg mixture. Stir in 3 cups of flour and ground cinnamon until a shaggy dough forms.

Continue adding remaining flour a little at a time until dough pulls away from the bowl.

Knead on a lightly floured surface for 5-7 minutes until smooth and elastic.

Place the dough in a lightly oiled bowl, turning once to coat. Cover with plastic wrap or a clean towel.

Let rise in a warm, draft-free place until doubled, about 1-1.5 hours.

Mix brown sugar, ground cinnamon, and melted butter until it forms a spreadable paste.

Punch down the risen dough. Roll it into a large rectangle on a floured surface.

Brush the rectangle with a little melted butter, then spread the cinnamon-brown sugar filling evenly across the dough.

Roll the dough tightly into a log, shape it into a ring, and pinch the ends to seal.

Place the ring on a baking sheet lined with parchment. Cover and let rise 30-45 minutes.

Preheat oven to 375°F (190°C) while it rises.

Bake the king cake for 25-35 minutes until golden brown.

Whisk together powdered sugar, milk or water, and vanilla until smooth.

Drizzle the glaze over the cooled cake and sprinkle sanding sugar in sections while glaze is still wet.

If using, hide a small plastic baby in a slice after baking or place it under the cake before serving.