Add the sugar, 1 teaspoon (5 mL) of milk, vanilla, cinnamon, nutmeg, and salt. Beat again, starting on low speed and covering the bowl or stand mixer with a tea towel, until smooth and fluffy, stopping to scrape down the bowl as necessary. If your buttercream is still too thick, you can thin it with a touch more milk. If it becomes too thin, you can thicken it by beating in more sugar.