In a microwave safe bowl, combine sugar, cornstarch, salt, and pumpkin pie spice Whisk dry ingredients together, then slowly add milk while whisking to prevent lumps. Add pumpkin puree and whisk again. Microwave on high for 3 minutes. Whisk. Microwave again in 1 minute intervals, whisking after each minute, until the pudding has thickened and is shiny (about 3-4 more minutes). Stir in the butter and vanilla. Place plastic wrap directly on top of the pudding to prevent a skin from forming. Refrigerate and serve cold.