For the bread
3 1/4 cups (455 g) Gluten Free Bread Flour, plus more for sprinkling
1/4 teaspoon cream of tartar
2 teaspoons (6 g) instant yeast
1/4 cup (50 g) granulated sugar
1/2 teaspoon (3 g) kosher salt
2 tablespoons (28 g) unsalted butter, at room temperature
8 ounces sour cream, at room temperature
1 egg (50 g, weighed out of shell) at room temperature, beaten
3 to 4 ounces warm water (about 95°F) (measure by weight, not by volume)
For the filling
4 tablespoons (56 g) unsalted butter, melted
1/2 cup (100 g) granulated sugar
2 teaspoons ground cinnamon
For the toppings
2 cups (230 g) confectioners’ sugar
1 tablespoon (14 g) unsalted butter, at warm room temperature
2 tablespoons (1 fluid ounce) milk, at room temperature, any kind (plus more by the 1/4 teaspoon if necessary)
About 3/4 cup (150 g) superfine sugar (or granulated sugar, pulsed a few times in a blender or food processor)
Green, Yellow, Blue and Red liquid food colorings
In the bowl of your stand mixer, place the flour, cream of tartar, instant yeast and sugar, and use a handheld whisk to combine well. Add the salt and whisk to combine well. Add the butter, sour cream, egg and 3 ounces water, and mix on low speed with the dough hook until combined. Raise the mixer speed to medium and knead for about 5 minutes. The dough should be a lovely, smooth, enriched dough. If necessary to achieve the desired texture, add the remaining 1 ounce of water and knead again until smooth. The dough should climb up the dough hook during kneading but remain intact and smooth. Spray a silicone spatula lightly with cooking oil spray, and scrape down the sides of the bowl. Transfer the dough to a lightly oiled bowl or proofing bucket large enough for the dough to rise to double its size, spray the top of the dough with cooking oil spray, and cover with an oiled piece of plastic wrap (or the oiled top to your proofing bucket). Place the dough in the refrigerator for at least 12 hours and up to 3 days.
Preparing the dough for shaping. On baking day, turn out the chilled dough onto a lightly floured surface and, using the scrape and fold kneading method and using a very light touch, sprinkle the dough with more flour and knead it lightly, sprinkling with flour when necessary to prevent it from sticking, scrape the dough off the floured surface with a floured bench scraper, then fold it over on itself. Repeat scraping and folding until the dough has become smoother. This dough should not need much work to become smooth. Do not overwork the dough or you will incorporate too much flour and it will not rise properly.
Shaping, filling and assembling the dough. Line a large, rimmed baking sheet with unbleached parchment paper and set it aside. On a lightly greased piece of unbleached parchment paper, using a floured rolling pin, roll the dough into a rectangle that is about 18-inches by 14-inches, and about 1/4-inch thick. In a small bowl, place all of the filling ingredients and mix to combine well. With a large, dull knife or offset spatula, spread the filling evenly over the entire surface of the dough, leaving a 1-inch border clean all around the perimeter. Using the parchment paper to aid you, beginning at one of the long sides, roll the dough tightly into a cylinder and pinch the seam and the ends closed securely. Transfer to the prepared baking sheet, seam side down, join the ends together and pinch them closed securely. Sprinkle the top and sides of the dough generously with bread flour to provide the dough a cloak to rise into, cover with greased plastic wrap and set in a warm, draft-free location to rise for about 40 minutes, or until it reaches about 150% of its original size. Do not overproof. As the dough is nearing the end of its rise, preheat your oven to 350°F.
Bake. Once the dough has finished rising, remove the plastic wrap and place the pan in the center of the preheated oven. Bake until lightly golden brown all over (about 30 minutes). Allow to cool for about 10 minutes on the pan before turning out onto a wire rack to cool completely.
Glaze and Decorate. While the bread is cooling, make the glaze and colored sugars. To make the glaze, mix the confectioners’ sugar, butter and 1 tablespoon milk into a thick paste. Add the remaining tablespoon milk one teaspoon at a time, mixing well, until you have a glaze that is smooth and thickly pourable. If it is very hard to stir, it needs another drop or two of milk. Set the glaze aside briefly. To make the colored sugars, divide the superfine sugar into 3 separate small bowls. To one bowl, add about 5 drops of yellow food coloring, and mix well, pressing the food coloring into the sugar with the back of a small spoon until the sugar is a uniform yellow color. To a second bowl, repeat the process with about 5 drops of green food coloring. To the final bowl, repeat the process with about 3 drops each of the red and blue food colorings (red + blue = purple). Once the bread is cool, mix the glaze (adding a few more drops of milk if necessary to return to the proper consistency) and then pour it over the top of the bread, allowing it to drizzle , and immediately decorate with the 3 different colored sugars in alternating bands. I used a small mesh sieve to distribute the sugars without their clumping. Slice and serve.