Pistachio Swiss Meringue Buttercream

Makes about 4 cups

4 large egg whites
1 cup sugar
1 cup (2 sticks) unsalted butter, softened, cut into tablespoons
Sweetened pistachio paste, to taste
Green food coloring (optional)

Put egg whites and sugar into the top of a double boiler over a pan of simmering water. Whisking constantly, cook until sugar has dissolved and mixture is warm (about 160 degrees).
Pour heated egg whites into the bowl of an electric mixer fitted with the whisk attachment. Beat egg white mixture on high speed until it forms stiff (but not dry) peaks. Continue beating until fluffy and cooled, about 7 minutes.
Switch to the paddle attachment. With mixer on medium-low, add butter two tablespoons at a time, beating well after each addition. Increase speed to medium-high; continue beating until frosting appears thick, about 3 minutes. Reduce speed to low; add 1/4 cup pistachio paste and mix until incorporated. Add more if a stronger flavor is desired (I used about 1/2 cup).

Sweetened Pistachio Paste
Makes about 1 cup

1 cup pistachios, skins removed (if pistachios are salted, rinse them well)
1/4 cup granulated sugar
1/2 cup hot water

In a food processor, grind pistachios and sugar until it becomes a fine meal. With the processor still running, Slowly add hot water, one tablespoon at a time scraping the bowl often, until a thick but spreadable paste forms. Transfer to a bowl and press plastic wrap against the surface. Cool to room temperature.

Credits: How to Eat a Cupcake blog