1. Make the dough: In the bowl of a standing mixer fitted with a hook, combine yeast, 1/2 tsp. of the sugar, and 1/4 cup water heated to 115°. Stir to combine and let sit until foamy, about 10 minutes. Add remaining sugar, milk, light brown sugar, vanilla, egg, and egg yolk. Beat on low speed until thoroughly combined, 1 minute. Turn mixer off and add flour and salt. Mix on medium speed until the dough just comes together. Turn mixer speed to high and knead dough for 4 minutes. Add the butter and continue kneading until dough is smooth and pulls away from the side of the bowl, about 6 minutes. Remove bowl from mixer, cover with plastic wrap, and let sit until doubled in size, 1 1/2–2 hours.
2. Meanwhile, make the filling: Combine cream cheese, brown sugar, pecans, maple syrup, cinnamon, salt, and zest in a large bowl and beat on medium speed of a hand mixer until combined; set aside.
3. Punch down dough and turn it out onto a heavily floured surface. Using a floured rolling pin, roll the dough into a large circle, about 1/4"-thick. Cut a hole in the center of the circle and pull with your fingers to widen. Place dollops of filling evenly around circle halfway between outer edge and inner hole. Drape outside edges over filling and continue rolling outside inward until filling is covered, widening inner hole as needed, until dough covers the seam. Transfer rolled dough circle to a parchment paper-lined baking sheet; cover with plastic wrap and let sit for 1 hour. Heat oven to 350°. Uncover cake and bake until golden brown, about 30 minutes. Let cool completely.
4. Make the icing: Whisk together the sugar and buttermilk in a small bowl until smooth. Transfer king cake to a cutting board or serving platter; spread icing evenly over top of cake and sprinkle evenly with sanding sugars.