Egg wash (1 large egg, beaten together with 2 Tbsp. water and a pinch of salt)
In a preferably nonstick saucepan, melt 2 tablespoons of the butter over high heat. Add the chicken and season with 1 teaspoon salt and a few grinds of pepper. Sauté, crumbling with a wooden spoon, until cooked through. Remove the chicken from the pan and set aside. Return the pan to the heat, add the remaining butter and the onion and cook over medium heat until the onion is soft and translucent. Add the curry powder and stir, then add the flour and cook, stirring constantly, for 3 minutes. Add the bay leaf, then add the stock in a slow stream, stirring constantly with a whisk. Return the chicken and any accumulated juices to the pan, along with the currants, and simmer until thickened, about 8 minutes. Season to taste with salt and pepper and set the mixture aside to cool completely.
Lay the pastry sheets on a lightly floured surface and roll them out into 10-inch squares. Cut into 2-inch squares. Brush the edges with the egg wash and place 1 teaspoon of the chicken mixture in the center of each. Fold in half to form a triangle, then press, removing as much air as possible to form a small sealed package. If desired, press with a fork to crimp the edges. Brush the tops with egg wash. (At this point, the triangles can be put in an airtight container and frozen for up to 1 month. Although you shouldn't thaw the triangles before baking, remember that it may take a few minutes longer for them to cook through if they've been frozen.)
Preheat the oven to 400°F. Put the triangles on baking sheets and bake until golden brown and cooked through, about 12 minutes. Serve hot or at room temperature.