2 ounces (4 tablespoons) unsalted butter, cut into small pieces
1 can (15 ounces) solid-pack pumpkin
1 can (12 ounces) evaporated milk
3/4 cup packed light-brown sugar
3 large eggs
1 tablespoon cornstarch
1 teaspoon pure vanilla extract
1 1/2 teaspoons coarse salt
3/4 teaspoon ground cinnamon
3/4 teaspoon ground ginger
1/4 teaspoon ground nutmeg
1 ounce milk chocolate, melted
Make the crust: Preheat oven to 350 degrees. Combine graham cracker crumbs, butter, sugars, salt, and cinnamon in bowl. Firmly press mixture into bottom and up sides of a deep, 9 1/2-inch pie dish. Bake until firm, 8 to 10 minutes.
Remove from oven, and sprinkle bittersweet chocolate over bottom of crust. Return to oven to melt chocolate, about 1 minute. Spread chocolate in a thin layer on bottom and up sides. Let cool on a wire rack. Reduce oven temperature to 325 degrees.
Make the filling: In a large heatproof bowl set over a pot of simmering water, melt semisweet chocolate and butter, stirring until smooth. Remove from heat.
Mix pumpkin, milk, brown sugar, eggs, cornstarch, vanilla, salt, cinnamon, ginger, nutmeg, and a pinch of cloves in a medium bowl. Whisk 1/3 pumpkin mixture into chocolate mixture. Whisk in remaining pumpkin mixture until completely incorporated.
Transfer pie dish to a rimmed baking sheet, and pour pumpkin mixture into crust. Bake until center is set but still a bit wobbly, 55 to 60 minutes. Let cool in pie dish on a wire rack. Refrigerate until well chilled, at least 8 hours (preferably overnight). Before serving, drizzle melted milk chocolate on top. Serve immediately.