In a medium saucepan over low heat, add the sour cream, sugar, butter, and salt. Cook until the butter has melted and the ingredients are thoroughly blended. Set the mixture aside, and allow it to cool.
In a small bowl, stir together yeast, warm water, and sugar. Allow the mixture to stand 5 minutes.
In a stand mixer fitted with a whisk attachment, beat the sour cream mixture, yeast mixture, eggs, and 2 cups bread flour at medium speed. Remove the whisk attachment from the standing mixer and replace it with a dough hook.
Gradually add the bread flour, about a cup at a time at low speed until a soft dough forms.
Next, flour a large work surface and then place the dough in the center. Knead the dough until smooth and elastic, about 10 minutes. Place the kneaded dough in a greased bowl. Cover and let rise in a warm place for 1 hour or until the dough has at least doubled in size.
Punch the dough down and turn out onto a floured work surface. Using a bench scraper, divide the dough. You will be making two separate king cakes. With a lightly floured rolling pin, roll each portion into a 22- x 12-inch rectangle.
Mix the cup of sugar with the cinnamon in a small bowl, then spread half of the softened butter on each rectangle of dough, leaving a 1-inch border. Evenly sprinkle the cinnamon-sugar mixture over the surface of each rectangle.
Using your hands, carefully roll each dough into one large log. Place each roll, seam side down on a lightly greased baking sheet. On the first roll, bring the ends of the roll together and form an oval ring. Pinch the ends together, and moisten lightly with water to seal. Repeat with the second roll. Cover and let rise in a warm place for 25 minutes or until doubled in size. Meanwhile, preheat your oven to 375 degrees.
Bake at 375ºF for 15-20 minutes or until golden brown. When cakes are slightly cool, drizzle with vanilla glaze and then sprinkle with Mardi Gras colored sanding sugars, alternating colors forming thick ribbons on the cakes.
In a medium bowl, add powdered sugar, melted butter, vanilla extract, and milk. Stir until smooth. Add more milk a teaspoon at a time until spreading consistency. If too thin, add more powdered sugar.
Substitute all-purpose flour for bread flour if you do not have bread flour on hand. Bread flour will give your king cake a chewier texture due to the higher protein content in the flour.
Place a small plastic baby under your cake. The baby represents the Christ child and will bring luck and prosperity to whoever receives the slice with the baby. It is also said, whoever receives the baby will have to bake the king cake for next year's celebration.
We no not recommend placing the plastic baby in the cake as it presents a choking hazard.
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If you're unable to find gold (yellow), purple, and green sanding sugar at your local grocer simply add 1 to 3 drops of food dye to 1/4 cup of granulated sugar in jar with a tight fitting lid. Shake vigorously.