Preheat oven to 350°F (175°C). Press the pie crust into a 9-inch pie plate. In a large bowl, whisk together pumpkin puree, sweetened condensed milk, eggs, pumpkin pie spice, and salt until smooth. Pour the mixture into the prepared pie crust and bake for 45-50 minutes, or until a knife inserted near the center comes out clean. Let the pie cool completely on a wire rack.

Preheat oven to 325°F (160°C). Grease and flour a 10-inch springform pan. In a medium bowl, whisk together flour, granulated sugar, baking soda, baking powder, cinnamon, ginger, nutmeg, and salt. In a separate large bowl, cream softened butter until light and fluffy, then beat in eggs one at a time, followed by buttermilk and vanilla extract. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Pour half of the spice cake batter into the prepared springform pan. Carefully place the cooled pumpkin pie on top of the batter in the center. Pour the remaining cake batter around and over the pie, ensuring the pie is mostly covered. Bake for 60-75 minutes, or until a wooden skewer inserted into the cake portion (not the pie) comes out clean. Let cool in the pan for 15 minutes before transferring to a wire rack to cool completely.

In a large bowl, beat softened cream cheese and butter with an electric mixer until smooth. Gradually add powdered sugar, mixing on low speed until combined, then beat on high speed until light and fluffy. Stir in vanilla extract, and add milk one tablespoon at a time if a thinner consistency is desired.

Once the Piecaken is completely cool, spread the cream cheese frosting evenly over the top and sides. For best results, chill the Piecaken for at least 2 hours before slicing and serving. Garnish as desired with additional spice or festive sprinkles.