Pistachio Buttercream Frosting

Preparation

1. Step 1

   Bring half and half and pistachios to boil in heavy small saucepan. Remove from heat; cover and let stand 1 hour. Whisk sugar, yolks and flour in medium bowl to blend. Bring half and half mixture to simmer; gradually whisk into yolk mixture.Return to same saucepan; stir over medium heat until mixture bubbles thickly, about 5 minutes. Mix in vanilla. Transfer to processor; blend until nuts are very finely chopped. Transfer pistachio pastry cream to bowl. Cover and cool to room temperature, about 2 hours.

   Step 2

   Using electric mixer, beat unsalted butter in large bowl until fluffy. Add pastry cream by 1/4 cupfuls, beating well after each addition. (Can be prepared 2 days ahead. Cover and refrigerate. Bring to room temperature before using.)