This homemade pumpkin buttercream frosting made with actual pumpkin puree is certain to get you in the fall spirit. This easy pumpkin spice frosting starts with a pastry cream base for a rich and creamy feel. Serve it on pumpkin cupcakes, chocolate cake, or as a dip for graham crackers! This recipe makes about 6 cups of frosting, so you'll have plenty for filling, frosting, and snacking!
In a medium heat-proof bowl, whisk sugar and cornstarch together until combined. In a separate medium bowl, whisk together pumpkin puree, eggs, vanilla, pumpkin pie spice, cinnamon, and salt. Add pumpkin mixture to sugar mixture, whisking until fully incorporated.
In a medium saucepan, bring milk to simmer over medium heat. Ladle 1/2 cup hot milk into the egg mixture and whisk to combine. Repeat with two more 1/2-cup additions, then pour egg mixture back into the saucepan.
Cook over medium heat, whisking constantly, until custard turns thick and lumpy, about 3-4 minutes. When custard begins to bubble, continue to cook and whisk exactly 2 minutes. Remove from heat and add vanilla and salt; stir to combine.
Spread the custard into a 13 in. x 9 in. baking dish. Let stand uncovered 25 minutes in the refrigerator or 45 minutes at room temperature, until custard cools to 68°F.
In a medium bowl, beat butter with an electric mixer fitted with paddle attachment at medium speed 5 minutes or until fluffy and light in color.
Stir the cooled custard until smooth. Add custard into the butter, one tablespoon at a time, mixing thoroughly until each is combined and scraping sides and bottom of the bowl often.
When the custard is fully incorporated, switch to whisk attachment and whip on medium speed 5 minutes or until the buttercream is light and fluffy.