01 - Preheat oven to 350°F and lightly grease a 9x13-inch baking dish.
02 - In a medium bowl, whisk together all-purpose flour, pumpkin pie spice, baking powder, and salt until evenly blended.
03 - In a large bowl, beat egg yolks with 3/4 cup sugar until pale and thick. Incorporate pumpkin puree, whole milk, and vanilla extract.
04 - Carefully fold the dry ingredient mixture into the wet ingredients just until combined.
05 - In a clean bowl, beat egg whites to soft peaks. Gradually add remaining 1/4 cup sugar and continue whipping until stiff peaks form.
06 - Gently fold whipped egg whites into the batter in three additions, preserving volume for light texture.
07 - Transfer batter to the prepared dish and smooth surface. Bake for 30 to 35 minutes, or until a toothpick inserted in the center emerges clean. Let cool in pan 15 minutes.
08 - While cake cools, whisk together sweetened condensed milk, evaporated milk, whole milk, and cinnamon until smooth.
09 - Poke holes throughout the warm cake using a fork or skewer. Gradually pour the milk mixture evenly across the surface. Allow to cool to room temperature, then refrigerate at least 4 hours or overnight.
10 - Before serving, whip heavy cream with powdered sugar and vanilla until soft peaks form.
11 - Spread whipped cream over the chilled cake. Dust lightly with cinnamon or pumpkin pie spice. Slice and serve.