Place pistachios in a food processor; process until a crumbly paste forms (about 3 1/2 minutes), scraping sides of bowl once.
Place 1/4 cup pistachio butter, 1 1/4 cups sugar, cocoa, eggs, and egg whites in top of a double boiler; stir well with a whisk. Add chocolate; cook over simmering water until chocolate melts and sugar dissolves (about 3 minutes). Remove from heat; add baking powder and vanilla. Stir with a whisk until smooth. Spoon batter into 12 muffin cups coated with cooking spray. Chill 2 hours.
Place remaining 1/4 cup pistachio butter and 1/2 cup sugar in food processor; pulse 4 times or until combined. Add milk and salt; process until smooth. Strain mixture through a sieve into a small saucepan; discard solids. Bring to a boil. Reduce heat; simmer 4 minutes or until thick. Remove from heat; pour into a bowl. Cover and chill.
Preheat oven to 450°.
Bake cakes at 450° for 9 minutes or until almost set (centers will not be firm). Let cool in pan 5 minutes. Invert each cake onto a dessert plate; drizzle with sauce. Garnish with powdered sugar, if desired.