01 - Preheat oven to 325°F. Grease and line a 9-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, whisk together pumpkin purée, eggs, granulated sugar, melted butter, and vanilla extract until smooth.
03 - Stir in 1/2 cup whole milk and 1/2 cup heavy cream until the mixture is well combined.
04 - Sift in all-purpose flour, salt, and pumpkin pie spice. Mix gently until just incorporated.
05 - Gradually whisk in lukewarm whole milk and heavy cream for the custard layer. Batter will be thin.
06 - Pour batter into prepared pan. Tap lightly on the counter to release air bubbles.
07 - Bake for 50 to 55 minutes or until the center is just set and the top turns golden brown. The cake will wobble slightly but set as it cools.
08 - Allow cake to cool completely in the pan, then refrigerate for at least 2 hours before slicing.
09 - Cut into squares and serve chilled, topped with whipped cream and a sprinkle of ground cinnamon or nutmeg as desired.