The secrets to this celestial buttercream, writes Vancouver baker Rosie Daykin in her first book, Butter Baked Goods (Random House, 2013), are "making sure your butter is not too cold, and then whipping it like crazy!" When properly whipped, buttercream is supremely fluffy--almost like whipped cream--and surprisingly un-greasy. "When it's right, it's a breeze to use," adds Daykin. "It's like pushing air around." Also, because her version has a lot less sugar than most buttercreams, it won't set your teeth on edge.
Makes 4 cups (enough to frost one 4-layer 8-in. cake) (serving size: 1/4 cup)
1. In a stand mixer fitted with the paddle attachment (or a handheld mixer with an eggbeater or whisk attachment), beat butter on high speed until as pale as ivory and very fluffy, at least 5 minutes. Scrape sides and bottom of bowl twice while beating.
2. Turn the mixer to low and gradually add sugar. When combined, scrape bowl again, then slowly drizzle in milk and vanilla; mix until combined. Mix in pistachio paste. If you'd like the frosting to be greener, in a small bowl, blend yellow and blue food colorings to make a green tint, scrape into frosting, and mix to blend. Scrape bowl again.
3. Turn the mixer to high and beat the buttercream until extremely fluffy, 8 to 10 minutes. "It should look almost like whipped cream, and should lose the waxy sheen that butter has."
Make ahead: Up to 3 hours at room temperature. Although you can make buttercream a few days ahead and chill, it will take a while to return it to room temperature (about 3 hours) and re-whip to the proper cloudlike texture--much longer than just making it fresh.