In a bowl, whisk the flour, baking powder, and salt together. Then combine with grated and chilled butter, until the mixture resembles coarse meal.
Slowly pour a few tablespoons of iced water, at a time, while mixing with your hands. Once the mixture begins to hold and appears crumbly, form a rough ball and cover with plastic wrap. Chill for at least 1 hour.
Meanwhile, make the mac and cheese: combine butter and flour in a pot. Slowly pour in cold milk, while stirring. Allow to thicken and stir in the cheeses. Season with salt and pepper, and then stir in the cooked elbow macaroni. Set aside.
Roll out the dough on a lightly floured surface, and cut out rounds.
Fill each round with 1-2 tablespoons of mac and cheese. Apply the egg wash around the edges, and then seal and crimp each one closed.
Brush the egg wash on the surfaces and sprinkle goldfish cracker crumbs on top.
Bake at 400°F for 15-20 minutes, or until golden brown.