Preheat the oven to 350°F (175°C). Line a cupcake pan with liners.

In a bowl, whisk together the flour, ground pistachios, baking powder, and salt.

In a separate bowl, cream the butter and sugar until light and fluffy. Add the eggs one at a time, then mix in the vanilla.

Alternately add the dry ingredients and buttermilk, mixing until just combined. Add green food gel if using.

Divide the batter evenly among the cupcake liners. Bake for 18-20 minutes, or until a toothpick inserted comes out clean. Let cool completely.

In a small saucepan, combine the sugar and water. Heat over medium heat until the syrup reaches 240°F (115°C) on a candy thermometer.

While the syrup cooks, whip the egg whites to stiff peaks in a stand mixer.

Slowly pour the hot syrup into the egg whites while whipping on high speed. Continue whipping until the meringue cools to room temperature.

Add the butter, one piece at a time, mixing until smooth. Add the vanilla and salt.

Pipe the buttercream onto the cooled cupcakes using a piping bag and tip.

Garnish with chopped pistachios.