01 - Combine warm milk and melted butter in a small bowl. Sprinkle yeast over the surface and let stand for 5 minutes until mixture becomes foamy.
02 - In a large bowl or stand mixer, combine the activated yeast mixture with granulated sugar, eggs, vanilla extract, and salt. Gradually incorporate flour, mixing until a soft, slightly sticky dough forms.
03 - Turn dough onto a lightly floured surface and knead for 6-8 minutes by hand, or use a stand mixer with dough hook for 5 minutes, until smooth and elastic.
04 - Place dough in a greased bowl, cover with plastic wrap or damp towel, and let rise in a warm, draft-free area until doubled in bulk, approximately 1 to 1.5 hours.
05 - While dough rises, combine brown sugar, ground cinnamon, flour, and softened butter in a small bowl. Mix until a spreadable paste forms.
06 - Punch down risen dough and roll into a 10x20-inch rectangle. Spread cinnamon filling evenly over surface, leaving a 1/2-inch border around edges.
07 - Starting from the long side, roll dough tightly into a log. Pinch seam to seal, then form into a ring by bringing ends together and pinching to close.
08 - Transfer ring to a parchment-lined baking sheet. Cover and let rise until puffy and expanded, about 45 minutes.
09 - Preheat oven to 350°F. Bake for 25-30 minutes until golden brown and cooked through. Cool completely on a wire rack before icing.
10 - Whisk together powdered sugar, milk, and vanilla extract until smooth and pourable consistency is achieved.
11 - Drizzle icing over cooled cake. Immediately sprinkle purple, green, and gold sanding sugars in alternating stripes while icing is wet.
12 - If using, insert plastic baby figurine into the underside of the cooled cake before serving.