In a small saucepan, gently warm the milk until it reaches a lukewarm temperature. Set it aside for later use.

Separate the egg whites from the yolks. In a clean, dry bowl, beat the egg whites with a pinch of cream of tartar until stiff peaks form. This will help create a light and fluffy texture in your cake.

Melt the butter in a separate bowl and cool to room temperature. This step ensures the butter won't cook the eggs when mixed.

In a large bowl, beat the egg yolks with the confectioner's sugar until the mixture becomes pale and frothy. This process helps to incorporate air into the batter, resulting in a more tender cake.

Add the cooled melted butter, vanilla extract, ground cinnamon, and pumpkin pie spice to the egg yolk mixture. Mix until well combined.

Gradually fold in the all-purpose flour, making sure not to overmix, as this can lead to a tough cake.

Add the pumpkin puree to the batter and 1/4 cup of the lukewarm milk. Stir until just combined.

Slowly pour in the remaining lukewarm milk with the mixer running on low speed. Mix until the batter is smooth and well combined.

Gently fold the beaten egg whites into the pumpkin batter using a spatula. Be careful not to overmix; you want to maintain the air bubbles in the egg whites for a light and fluffy cake.

Pour the batter into the prepared baking dish and place it in the oven. Bake for 60-65 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Once baked, remove the cake from the oven and allow it to cool completely. For best results, refrigerate the cake before cutting, as this will help the custard layer set properly and prevent a runny texture.

When ready to serve, cut the Pumpkin Magic Custard Cake into squares and enjoy the delightful combination of creamy custard and fluffy cake in every bite.