Preheat your oven to 390℉.
Make your yogurt egg wash. Melt the butter and let it cool slightly. Then, whisk together the egg, yogurt, and melted butter into a creamy, slightly thick sauce-like mixture.
On parchment paper, lay out one sheet of phyllo. Dot and carefully brush some of the yogurt wash across this, being mostly thorough but careful not to get too close to the edges that you soften and make the phyllo stick.
Layer up four layers of phyllo with wash in between each layer. Phyllo - Wash - Phyllo - Wash - Phyllo - Wash - Phyllo - Wash. This will make one log.
Using about a third of the filling, make a pile along the long edge of the layered phyllo.
Using the parchment paper, roll the log by pulling the parchment over. Make sure it fully rolls, though ideally it shouldn't stick to the paper. It doesn't need to be too tight.
Repeat this process two more times, for a total of three logs, each made from four layers of phyllo and about a third of the filling.
Now make the spiral (if you want; you can bake the logs as-is on a baking sheet instead of spiraled in a cast iron if you like.) Start with one and roll it horizontally. Take the second, tuck one end onto the open edge of the first, and continue the spiral; repeat with the third.
Brush the bottom of your cast iron with melted butter, a generous layer to help prevent the pastry from sticking when it's done.
Transfer the spiral into the cast iron. If it's stuck to the parchment paper, you can put the cast iron upside down over the top of the spiral, slide a cutting board underneath the parchment paper, and flip it. If the phyllo tears when you remove the parchment, you can patch it with more phyllo, though ideally you shouldn't need to do any of this and it should transfer nicely.
Brush the top of the spiral with more melted butter, and bake for 35-40 minutes, or until the phyllo is golden and crisp and the filling is warmed up. Slice and enjoy with the yogurt sauce!