While the cake is cooling, prepare pistachio buttercream frosting. In large mixing bowl with paddle attachment, or using a handheld electric mixer, beat the butter and pistachio paste until fluffy, about 2 minutes. Add the powdered sugar, heavy cream and vanilla extract and beat until smooth and fluffy, about 2 minutes. Transfer the frosting into a piping bag with a round tip (I used
Wilton 1A decorating tip).