01 - Combine warm milk and yeast in a small bowl. Let stand for 5 minutes until mixture becomes foamy, indicating the yeast is active.
02 - In a large mixing bowl, beat softened butter and granulated sugar until light and fluffy. Add eggs one at a time, mixing thoroughly after each addition. Blend in salt, nutmeg, vanilla extract, and lemon zest.
03 - Pour the activated yeast mixture into the butter mixture. Gradually incorporate flour, mixing until a soft dough forms. Knead by hand for 8 minutes or use a stand mixer with dough hook for 5 minutes until smooth and elastic.
04 - Place dough in a greased bowl, turning once to coat all surfaces. Cover with plastic wrap or a clean towel and let rise in a warm, draft-free area until doubled in bulk, approximately 1.5 hours.
05 - While dough rises, combine brown sugar and ground cinnamon in a small bowl. Mix thoroughly until evenly incorporated.
06 - Punch down risen dough and transfer to a lightly floured surface. Roll into a 10x20-inch rectangle. Brush entire surface with melted butter, then sprinkle cinnamon-sugar mixture evenly across the dough.
07 - Starting from a long side, roll dough tightly into a log. Pinch seam firmly to seal. Transfer log to a parchment-lined baking sheet and shape into an oval or circle, pinching ends together to form a ring. Tuck plastic baby underneath if using.
08 - Cover shaped cake loosely and let rise in a warm place until puffy and noticeably increased in size, 30-45 minutes.
09 - Preheat oven to 350°F. Bake for 25-30 minutes until golden brown and cooked through. Transfer to a wire rack and cool completely before icing.
10 - Whisk powdered sugar, milk, vanilla extract, and salt in a small bowl until smooth and pourable. Adjust consistency with additional milk if too thick or more sugar if too thin.
11 - Drizzle icing generously over cooled cake, allowing it to drip down the sides. Immediately sprinkle colored sugars in alternating bands of purple, green, and gold while icing is still wet.