Preheat oven to 325°F (163°C). Grease a 9×13 inch baking dish with butter. Beat egg whites until stiff peaks form. Set aside in a large bowl. In another bowl, whisk egg yolks with 3/4 cup sugar until pale and thick. Mix in melted butter, pumpkin puree, and vanilla extract until smooth. Sift together flour, salt, pumpkin pie spice, and cinnamon. Gradually whisk into pumpkin mixture. Slowly add lukewarm milk, whisking constantly until smooth batter forms. Gently fold in beaten egg whites in three additions until just combined. Pour batter into prepared baking dish and bake for 50-60 minutes until set and golden. Cool completely at room temperature, then refrigerate for at least 4 hours. Dust with powdered sugar and serve with whipped cream.