01 - Combine warm milk and yeast in a small bowl. Let stand for 5 minutes until foamy and activated.
02 - In a large mixing bowl, beat butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each addition. Blend in salt, nutmeg, vanilla, and lemon zest.
03 - Pour in the yeast mixture and gradually incorporate flour. Knead until a soft, smooth dough forms, approximately 8 minutes by hand or 5 minutes with a stand mixer using dough hook.
04 - Place dough in a greased bowl, cover with plastic wrap or towel, and let rise in a warm, draft-free area until doubled in size, about 1.5 hours.
05 - In a small bowl, combine brown sugar and ground cinnamon until evenly mixed.
06 - Punch down risen dough and roll out on a floured surface to a 10x20-inch rectangle. Brush melted butter evenly over surface, then sprinkle with cinnamon-sugar mixture.
07 - Starting from the long side, roll dough into a tight log. Pinch seam to seal. Transfer to parchment-lined baking sheet and shape into a ring, pinching ends together securely.
08 - Cover loosely and let rise until puffy, 30-45 minutes. Optionally tuck plastic baby into dough from underneath before baking.
09 - Preheat oven to 350°F. Bake for 25-30 minutes until golden brown and cooked through. Cool completely on wire rack before icing.
10 - Whisk together powdered sugar, milk, vanilla, and pinch of salt until smooth and pourable. Adjust consistency with additional milk if needed.
11 - Drizzle icing evenly over cooled cake. Immediately sprinkle colored sugars in alternating sections of purple, green, and gold while icing is wet.