Step 1Make the filling: In a large skillet over medium heat, melt the butter. Stir in the diced apple, brown sugar, cinnamon and salt, and cook, stirring frequently, just until the apple starts to soften, 4 to 5 minutes (the pieces should still be crisp). Remove from heat and stir in the raisins. Spread the apple mixture onto a baking sheet to allow the apples to cool quickly, then cover and refrigerate until needed. The filling can be made up to 2 days in advance.
Step 2Heat the oven to 375 degrees.
Step 3Fill the pie shells: Brush the edges of each bottom crust with the prepared egg wash and fill with the apple filling, a generous one-fourth cup filling for each shell, carefully packing it in the shell so there are no gaps and mounding the filling slightly in the center. Top the filling with the top crusts (use a knife to cut vent holes in the top crusts if they are not already vented), and brush the top crusts with egg wash. Gently roll the outer edges of each bottom crust inward over the top crust to seal the pies and crimp as desired. Brush with a little more egg wash and dust if desired with a sprinkling of coarse sugar.
Step 4Bake the mini pies, one tin at a time, in the center of the oven until the filling is set and the crust is puffed and golden brown, about 35 minutes.
Step 5Cool the pies, still in the tins, to room temperature, twisting them gently every few minutes so they do not stick to the pan.