Preheat your oven to 325°F (163°C) and line an 8x8 or 9x9 inch square baking dish with parchment paper.

Gently warm the whole milk over low heat until it reaches a lukewarm temperature, about 5 minutes.

Beat the egg whites in a clean bowl until foamy, then gradually add cream of tartar and whisk until stiff peaks form, approximately 3-5 minutes.

Melt the butter in a microwave-safe bowl and allow to cool slightly.

In a separate bowl, combine egg yolks and confectioner’s sugar, beating until pale and frothy. Stir in melted butter, vanilla, cinnamon, and pumpkin pie spice.

Sift in all-purpose flour and mix gently. Then add pumpkin puree and 1/4 cup of lukewarm milk, stirring until just incorporated.

Gradually mix in the remaining lukewarm milk on low speed until completely combined.

Fold the beaten egg whites into the batter gently, maintaining some air in the mixture.

Pour the batter into the prepared baking dish and bake for 60-65 minutes until the top is golden brown and slightly jiggly.

Allow the cake to cool completely on a wire rack. Refrigerate for a couple of hours or overnight before serving.