In a saucepan over medium heat add the remaining blackberries, salt, lemon juice, and sugar. Cook until the blackberries start releasing their juice. Once the blackberries have been cooking for about 4-5 minutes add the cornstarch mixture and cook until the compote thickens. Set aside and allow to cool to room temperature.
While the compote is cooling roll out the puff pastry and cut into 18 equal squares. Make a slice starting about ⅔ from the center towards and through each corner and place on the lined sheet pan.
Spoon 1 teaspoon of the blackberry compote into the middle of each of the squares and fold the left side of each corner down towards the center.
Mix the egg yolk with the milk and brush over the tops of the pinwheels. Place a blackberry on the top of each pinwheel.
Bake for 10-15 minutes or until the crust is a golden brown.
Dust with the powdered sugar right before serving.