Preheat oven to 300F. Fill a shadow oven-proof pan with water and place on the lower oven rack. Spray a 10-inch springform pan with nonstick spray. Line bottom with parchment paper and spray again.
In a bowl of a stand mixer fitted with a whisk attachment, beat eggs, vanilla extract, and sugar at high speed until the mixture becomes stiff and light in color, about 10 minutes.
Meanwhile in a medium pan, melt the butter and chocolate over low heat, stirring until all melted. Add pumpkin, cinnamon, ginger, nutmeg, and salt. Stir to combine well.
Add pumpkin mixture to egg mixture and whisk together quickly and thoroughly. Pour mixture into prepared pan. Bake for 1 hour. Remove from the oven and let cool completely.
Cover with aluminum foil and refrigerate for at least 4 hours. Before serving, remove from pan and sprinkle with confectioner’s sugar, if desired. Refrigerate in an airtight container for up to 3 days.