- Line 2 large baking sheets with parchment.
- Whirl 1 1/2 cups icing sugar with pistachios in a food processor until very fine. Pass through a sieve into a medium bowl. Return any coarse nuts to food processor and whirl again. Repeat sieving. Discard any nuts remaining. Set aside.
- Beat egg whites in the bowl of a stand mixer on medium until frothy, about 30 sec. Add granulated sugar in a thin, steady stream. Increase speed to medium-high and beat until soft peaks form when beaters are lifted, about 1 min.
- Fold half of pistachio mixture into egg whites until almost combined. Gently fold in remaining mixture until no white streaks remain. Mixture will look like thick, moist cake batter.
- Fit a piping bag with a #7 round tip. Fill bag with batter. Pipe 1-in.-wide rounds onto prepared sheets 1/2 in. apart. If piped rounds have peaks, spread a kitchen towel on your counter. Smack the bottom of the sheets on kitchen towel a few times until tops are smooth. Let macarons rest until the surface of the batter feels dry when touched, 45 min to 1 hour (batter should not stick to your finger). It may take longer if the weather is humid.
- Preheat oven to 280F.
- Bake macarons in centre of oven, 1 sheet at a time, until they are firm and dry, 16 to 18 min. They should not shift when they are touched. If macarons are not done, continue baking and checking at 2 min intervals. Transfer to a rack to cool completely. Cookies should come off the parchment easily when cool.
- Beat 1/2 cup icing sugar with butter and vanilla in a medium bowl with an electric mixer on medium-high, until smooth. Set aside.
- Spread or pipe 1/2 tsp vanilla buttercream on the bottoms of half of the macarons. Sandwich with remaining macarons. Macarons will keep well, stored in an airtight container at room temperature, up to 2 days.
For best results, all ingredients need to be at room temperature.
Keep sandwiched macarons in an airtight container in freezer for up to 2 weeks. Let macarons come to room temperature overnight before serving.
If your piped rounds are larger than 1 in. you will get fewer macarons. Larger macarons will need a longer baking time.