How to Make It
Stir together flour, salt, and sugar in a large bowl. Add cubed butter, and, using your fingers, rub into flour mixture until pea-size pieces form. Stir in egg until just incorporated. Drizzle with evaporated milk, and mix with your hands until clumps form. Turn mixture out onto a lightly floured surface, and knead just until able to shape into a disk; wrap disk tightly in plastic wrap, and refrigerate 30 minutes. Roll dough to 1/8-inch thickness on a lightly floured surface, and fit into a 9-inch pie plate. Trim off excess dough; crimp edges as desired. Refrigerate at least 1 hour or up to 4 hours.
While pie crust chills, place sweet potatoes in a large saucepan filled with cold water. Bring to a simmer over medium-high, and cook until tender when pierced with a paring knife, 40 to 50 minutes. Drain sweet potatoes, and chill until cool, about 1 hour. Peel sweet potatoes, and mash with a potato masher until mostly smooth. Set aside 2 cups mashed sweet potatoes. (Reserve any remaining mashed sweet potatoes for another use.)
Preheat oven to 350°F. Whisk together mashed sweet potatoes, sugar, and melted butter in a large bowl until smooth. Whisk in eggs, evaporated milk, flour, nutmeg, cinnamon, ginger, and salt until thoroughly blended. Pour filling into prepared pie crust. Bake in preheated oven until filling is slightly puffed and set, about 40 minutes. Transfer pie to a wire rack, and let cool completely, about 4 hours.
Just before serving, stir together honey and lemon juice in a medium bowl. Add apple and squash, and gently toss. Serve pie with Honeycrisp-kabocha salad and licorice ice cream, and garnish with lemon balm leaves.