Step 1

Make the dough: In the bowl of a stand mixer fitted with the whisk attachment, combine the yeast, 1/2 teaspoon granulated sugar and 1/4 cup water on medium speed until the yeast dissolves, about 2 minutes. If necessary, whisk by hand to reach down deep into the bowl. Let sit until foamy, about 10 minutes.

Step 2

Add the remaining granulated sugar, milk, light brown sugar and vanilla. In a small bowl or liquid measuring cup, lightly beat together the egg and egg yolk and add to the mixer bowl. Beat the mixture on low speed until thoroughly combined, about 2 minutes.

Step 3

Turn the mixer off and switch to the dough hook attachment. Add the flour and salt. Mix on medium speed until the dough just comes together, about 1 to 2 minutes, scraping down the sides of the bowl. Then, knead the dough for 3 to 4 minutes. Add the butter and continue kneading until the dough is smooth and all the butter is incorporated. The dough should begin to pull away from the sides of the bowl after about 6 minutes. If the dough does not pull away, use a rubber spatula to scrape down the sides of the bowl, forming the dough into a ball.

Step 4

Remove the bowl from the mixer, cover with a clean kitchen towel, and place in a warm spot to rise until the dough has doubled in size, 1 1/2 to 2 hours.

Step 5

Make the filling: In a small bowl, combine the melted and cooled butter, softened cream cheese, confectioners' sugar, cinnamon and lemon zest. Whisk until thoroughly combined. The filling should be glossy and easily spreadable.

Step 6

Shape the cake: Punch down the dough. Place it on a heavily floured surface. Using a floured rolling pin, roll the dough into a large circle, 16 to 18 inches in diameter and 1/4-inch thick. Punch a hole in the center of the circle with your finger. Then widen the circle to about 3 inches. Using the back of a spoon or an offset spatula, spread the filling evenly around the ring of dough, halfway between the outer edge and inner circle, leaving about a 1-inch border on each side.

Step 7

Gradually fold the outside edge of the dough over the filling to meet the inner edge, continuing until the filling is covered, widening the center hole as you go. The hole should be about 8 inches wide when finished. Make sure the seams of the dough are well sealed by gently pinching the dough as needed. If necessary, dampen your fingers a bit and pinch to seal the dough. This prevents the filling from seeping out during baking.

Step 8

Line a rimmed baking sheet with parchment paper and place it next to the dough circle. Lift one end of the dough circle and place it on the sheet pan; then lift the other end onto the pan. Gently reform the dough into an oblong or rectangle on the pan, leaving at least an inch or two from the rim. Check to see that all seams remain sealed. Cover with a clean kitchen towel. Let it rest for 1 hour at room temperature until slightly puffed.

Step 9

Half an hour before the cake finishes proofing, position the rack in the middle position and preheat the oven to 350 degrees. Remove the towel from the cake. Bake until golden brown and dry to the touch, 20 to 25 minutes. Using the parchment paper, lift the cake from the sheet pan and move it to a wire rack. Let the cake cool completely before decorating, at least 1 hour.

Step 10

To decorate: While the cake is cooling, make the glaze. In a medium bowl, combine the confectioners' sugar, yogurt, lemon zest and vanilla. Add the buttermilk or milk gradually, whisking until the glaze is smooth and fluid enough to drizzle over the cake. Add more liquid, 1 teaspoon at a time, as needed to achieve the proper consistency.

Step 11

Line a serving platter with wax or parchment paper. Transfer the cake to the platter.

Step 12

Drizzle the glaze generously onto the cake, allowing it to drip down the sides. Sprinkle the cake with the sanding sugars, alternating strips of purple, green and gold. Allow the glaze to set for about 10 minutes. Gently lift the decorated cake and slide it off the paper on to a platter.

Step 13

Slice and serve.