In a medium sized bowl, whisk together flour and salt. Add in butter cubes and, using your hands, work the butter into the flour until small, pea-sized clumps form. Pour in buttermilk and sprinkle in thyme. Work liquid into flour until a shaggy dough forms. Knead slightly, form into a disc and cover with plastic wrap. Refrigerate while you make the filling. Dough can be made one day ahead of time.
For the Filling
Preheat oven to 400F.
Heat olive oil in a 10 inch cast iron pan over medium-high heat. Add in chicken pieces and cook until they start to brown, stirring occasionally. Transfer chicken to plate and set aside.
Add onions to pan and cook until they begin to brown, about 4 minutes. Add in carrots and cook until they soften, about 10 minutes. Stir in garlic and kale and cook until kale starts to wilt, about 2 minutes. Season with salt.
Reduce heat to low and sprinkle flour and rosemary on top of vegetables. Stir constantly for about 2 minutes, until flour is well integrated. Add broth and squash, bring to a boil, reduce heat, and simmer for about 10 minutes, until squash has softened.
Return chicken to pan, stir, and season filling with salt to taste. Turn off heat.
To Assemble and Bake:
- On a lightly floured surface, roll out dough to a 13 inch round. Gently drape dough on top of cast iron pan, tucking the sides in and crimping them with a fork. Cut four 1 inch slits in the center of the dough to vent, then brush with egg wash.
- Bake pot pie for 20-25 minutes, until crust is golden brown. Let cool for 10-15 minutes before serving.