1. Prepare crust. Cut butter into cubes. Place flour, sugar, salt, and cinnamon in food processor bowl; pulse 3–4 times until combined. Add butter; pulse 5–6 times until crumbly. Drizzle in water, 1 tablespoon at a time, pulsing until dough begins to form (may not use all water). Form dough into a disc, cover with plastic wrap, and chill at least 30 minutes.
2. Preheat oven to 375°F. Dust work surface lightly with flour. Roll out dough disc into ⅛-inch-thick, 12-inch-diameter circle. Roll crust carefully around rolling pin, then unroll gently into 9-inch pie dish without stretching; trim excess dough to ½ inch past dish edge, if needed. Use kitchen shears to snip ½-inch slits into edge of dough about ½ inch apart to make tabbed edge. Press every other tab toward center of crust. Place pie dish on baking sheet to catch any drips.
3. Prepare filling. Melt butter; beat eggs. Combine butter, eggs, pumpkin, and milk in large bowl until blended. Stir in pie spice, vanilla, and salt. Pour filling carefully into piecrust. Bake on middle oven rack 50–55 minutes just until filling is set. Cool on wire rack 1 hour, then chill 1 hour.
4. Prepare topping. Line baking sheet with parchment paper. Heat sugar in medium saucepan on medium-high 3–5 minutes without stirring until sugar begins to melt. Reduce heat to low; cook 3–5 minutes, stirring frequently with wooden spoon, until melted and golden. Pour syrup quickly and carefully onto parchment and spread into thin layer using back of spoon. Let stand 15 minutes to cool, then crack into shards using handle of spoon. Serve pie topped with sugar shards and whipped cream, if using.

Chef's Tip: Cover edge of piecrust with foil during last 15 minutes of baking to prevent overbrowning.