Sift together the all-purpose flour and baking powder. In a saucepan, melt dark chocolate and unsalted butter over low heat, cool slightly. Whisk eggs, granulated sugar, vanilla extract, and salt until pale and fluffy for 8-10 minutes. Mix melted chocolate with egg mixture, then fold in dry ingredients and add chopped chocolate and pistachios. Let rest for 10-15 minutes.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop tablespoons of dough onto sheets, bake for about 10 minutes or until puffed and crackly but soft in center. Let cool on sheets before transferring to a wire rack.

Sift pistachio pudding mix into a bowl, whisk in milk and let sit for 5 minutes. Mix softened butter, powdered sugar, vanilla extract, salt, and almond extract with pudding mixture. Beat until light and fluffy, adjust consistency with more powdered sugar if needed.

Spread pistachio frosting on the bottom of one cooled cookie and top with another to create a sandwich. Roll edges in chopped pistachios if desired. Repeat with remaining cookies and frosting.