1. Combine yeast, 1/2 teaspoon granulated sugar, and 1/4 cup warm water in a small bowl; let stand 5 minutes, and set aside. Place flour, 5 tablespoons granulated sugar, zest of 2 tangerines (about 2 teaspoons), and salt in bowl of a stand mixer fitted with a paddle. Mix to combine.
2. Heat milk in a small saucepan over medium-low 1 to 2 minutes or until warm to touch. Add 6 tablespoons butter, and allow to melt. Remove from heat.
3. Add 2 eggs, 1 egg yolk, dissolved yeast, and milk mixture to flour mixture. Mix on low speed 6 to 8 minutes or until a soft, tacky dough forms, stopping once or twice to push dough off paddle and sides of bowl. (Dough should just start to clear sides of bowl and feel satiny.) Add a few sprinkles of flour, if necessary. Transfer dough to a large greased bowl, cover with plastic wrap, and let stand in a warm place about 1 1/2 hours or until doubled in bulk.
4. Grease 2 (9-inch) round cake pans. Mix remaining granulated sugar (about 1/2 cup) and cinnamon in a small bowl. Turn dough out onto a lightly floured surface, and press to deflate. Roll into a 17- x 11-inch rectangle with longer edge toward you. Brush with melted butter, and sprinkle with cinnamon sugar. Roll up dough lengthwise, pinching seam to seal.
5. Cut dough into 12 equal slices with a large sharp knife, and place slices in prepared pans. Cover and let rise in a warm place until almost doubled in bulk, about 30 to 45 minutes. Preheat oven to 350°.
6. Brush surface of buns with egg wash. Bake at 350° for 20 to 22 minutes or until golden brown. Remove from pans, and let cool 5 minutes.
7. Whisk together powdered sugar, vanilla, tangerine juice, and remaining 1 teaspoon zest, and drizzle over warm rolls. Serve warm or at room temperature.