01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - Process 4.25 oz pistachios in a food processor until finely ground. Reserve 1-2 tbsp for garnish.
03 - Sift together flour, baking powder, and salt. Stir in the ground pistachios until evenly distributed.
04 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 3 minutes.
05 - Add eggs one at a time, beating thoroughly after each addition. Mix in vanilla extract.
06 - Add dry ingredients in two batches, alternating with milk. Begin and end with the flour mixture, mixing until just incorporated.
07 - Distribute batter evenly among prepared cupcake liners, filling each about two-thirds full.
08 - Bake for 18-20 minutes until a toothpick inserted into the center comes out clean. Allow to cool completely before frosting.
09 - Grind 3.5 oz pistachios finely, leaving slightly coarse texture preferred. Beat butter until smooth and creamy.
10 - Gradually incorporate powdered sugar, beating until combined. Add ground pistachios, vanilla, salt, and 2 tbsp milk. Beat until fluffy, adjusting consistency with additional milk if needed.
11 - Pipe or spread buttercream onto cooled cupcakes. Sprinkle with reserved ground pistachios for garnish.