In the bowl of a stand mixer, fitted with the dough hook, combine the flour, yeast, sugar and salt. Add the milk and butter and beat on low speed until the dough starts to come together. Increase speed to medium, and beat until a soft dough forms, about 10 minutes.

Shape the dough into a ball, place in a greased bowl, and turn to coat both sides. Cover loosely with plastic wrap and a clean towel, and place in a dry, warm place. Let stand until dough has doubled in size, about 1 hour.

Turn dough out onto a lightly floured work surface, and divide into 6 even portions. Roll each piece into a ball. Place balls on a baking sheet, cover loosely with a kitchen towel and let stand in a dry, warm spot until slightly springy, about 20 minutes.

Meanwhile, let's make the filling. Place the thawed spinach into a clean kitchen towel and wring out as much of the liquid as possible. Transfer to a large bowl and stir in the artichoke, cream cheese, sour cream, mayonnaise, parmesan, feta, mozzarella, pepper, granulated garlic, granulated onion, and crushed red pepper flakes until well combined.

Preheat oven to 450°F. Line two baking sheets with parchment paper and set aside.

Lightly dust a work surface with flour. Working 1 dough ball at a time, roll into an oval shape with a maximum length of 10 inches and maximum width of 4 inches. Roll dough borders up and toward center of oval, and pinch dough together at ends to create a canoe shape. Spoon about 1/2 cup of the spinach artichoke mixture into the center and spreading it out to the edges. Repeat with remaining dough balls and cheese mixture.

Place three khachapuri on each prepared baking sheet, spacing them out evenly. Brush the edges with 1/3 of the melted butter and bake in preheated oven for 15 minutes. Carefully remove baking sheets from the oven. Place an egg yolk in the center of each khachapuri. Brush crusts evenly with 1/3 more of the melted butter. Return to oven, and bake until dough is golden brown and yolks have set, about 15 minutes longer. Remove from oven once more and sprinkle with a bit of salt and pepper, and brush evenly with the remaining butter. Serve immediately and enjoy!