01 - In a small bowl, combine the warm milk and water. Sprinkle the active dry yeast and 1 tablespoon of granulated sugar over the top. Allow to stand until foamy, approximately 5 to 10 minutes, indicating the yeast is active.
02 - In a large mixing bowl, beat the softened butter with the remaining granulated sugar until light and fluffy. Incorporate the eggs, vanilla extract, lemon zest (if using), ground nutmeg, and salt, mixing thoroughly.
03 - Add the activated yeast mixture to the wet ingredients. Gradually add half of the all-purpose flour, stirring until just incorporated. Continue to add the remaining flour, mixing until a soft dough forms.
04 - Transfer the dough to a lightly floured surface and knead for 8 to 10 minutes until it becomes smooth and elastic. Place the dough in a greased bowl, cover with plastic wrap or a clean kitchen towel, and let rise in a warm spot until it has doubled in size, which typically takes about 1 hour.
05 - While the dough undergoes its first rise, prepare the cinnamon filling. In a small bowl, combine the packed brown sugar and ground cinnamon. Set aside the 1/4 cup of softened butter for later use.
06 - Once the dough has risen, gently punch it down to release the air. On a lightly floured surface, roll the dough into a rectangle approximately 10 by 20 inches.
07 - Evenly spread the 1/4 cup of softened unsalted butter over the entire surface of the rolled dough. Then, sprinkle the cinnamon-sugar mixture evenly over the butter.
08 - Starting from one of the long edges, tightly roll the dough into a log. Pinch the seam firmly to seal it closed.
09 - Carefully form the dough log into a ring, bringing the ends together and pinching them securely to seal. Transfer the shaped ring to a baking sheet lined with parchment paper. Cover loosely and allow to rise in a warm place until visibly puffy, about 30 to 40 minutes.
10 - Preheat your oven to 350°F. Bake the King Cake for 25 to 30 minutes, or until the top is golden brown and the cake is cooked through. Let the cake cool completely on a wire rack before proceeding with decoration.
11 - For the icing, whisk together the powdered sugar, 2 to 3 tablespoons of milk, and 1/2 teaspoon of vanilla extract until the mixture is smooth and has a pourable consistency.
12 - In separate small bowls, mix a few drops of each food coloring (purple, green, and gold/yellow) into the divided granulated sugar until each portion of sugar is vibrantly colored.
13 - Once the cake is completely cool, drizzle the prepared icing generously over the top. Immediately, while the icing is still wet, sprinkle the colored sugars in alternating sections (purple, green, gold) to create the festive Mardi Gras design.
14 - (Optional) For a traditional King Cake, hide a small plastic baby or a dried bean inside the cake after it has been baked and decorated. The person who finds it is believed to have good luck and is often designated as the host for the next King Cake gathering.