Make the Dough
In a small bowl, combine warm water, yeast, and 1 tablespoon of the sugar. Let sit for 5–10 minutes until foamy.
In a large bowl, whisk together flour, remaining sugar, salt, nutmeg, and cinnamon.
Stir in warm milk, melted butter, eggs, vanilla, and the yeast mixture.
Mix until a sticky dough forms, then knead on a floured surface (or with a stand mixer and dough hook) for 8–10 minutes until smooth and elastic.
First Rise
Place dough in a greased bowl, cover, and let rise in a warm place until doubled in size about 1 to 1 ½ hours.
Shape and Fill
Punch down the dough and roll into a large rectangle (about 10x20 inches).
Spread softened butter over the surface, then sprinkle evenly with brown sugar and cinnamon.
Roll the dough tightly from the long side into a log.
Shape into a ring, pinching the ends together to seal. Place on a parchment-lined baking sheet.
Second Rise
Cover and let rise for another 45 minutes, or until puffy.
Bake
Preheat oven to 350°F (175°C).
Bake for 25–30 minutes, or until golden brown.
Let cool completely before glazing.
Glaze and Decorate
In a bowl, whisk together powdered sugar, milk, and vanilla until smooth.
Drizzle over cooled cake, then immediately sprinkle with alternating sections of purple, green, and gold sanding sugar.
If using a trinket, carefully hide it in the cake after baking and cooling.