Preheat your oven to 350°F (175°C).
In a food processor, grind pistachios until finely ground.
Cream butter and powdered sugar until light and fluffy.
Beat in eggs one at a time, then add almond extract.
Fold in ground pistachios to create the filling.
Slice croissants horizontally without cutting all the way through.
Fill each croissant generously with pistachio mixture.
Place croissants on a baking sheet and brush tops with milk.
Bake for 12-15 minutes until golden and heated through.
Dust with powdered sugar before serving.