Step 1 Place the sliced apples and the apple cider in a large saucepan over medium heat. Cover the pan and let the apples steam in the cider until tender, 8 to 10 minutes. The apples should be soft, but still hold their shape.
Step 2 Strain the apples and let cool to room temperature.
Step 3 Generously butter the insides and tops of a twelve-cup standard muffin pan and coat each muffin cup well with granulated sugar, tapping out any excess.
Step 4 In a small bowl, mix together 1/2 cup [100 g] of the sugar, the cinnamon, and salt.
Step 5 Generously flour your work surface and roll out the croissant dough into a 10 by 24 in [25 by 60 cm] rectangle. Brush the dough with the melted butter, then sprinkle the sugar mixture evenly over the dough, gently pressing it into the butter to adhere. Scatter the cooled, strained apples over the sugar. Starting at a long side, roll up the dough into a tight cylinder and position the dough seam-side down. Cut the dough into twelve pieces, each measuring 2 in [5 cm] wide. Transfer the pieces to the prepared muffin pan and place them cut-side down. Cover the pan loosely with plastic wrap and let the dough rise until doubled in size and puffy (it should act similarly to a marshmallow with pressed), 2 to 2 1/2 hours.
Step 6 Adjust an oven rack to the middle position and preheat the oven to 400F [200C]. Line a sheet pan with parchment paper and set a wire rack inside of it.
Step 7 Remove the plastic wrap and gently press down on the top of each bun with a greased spatula. Place a sheet pan on the lower oven rack (this will help catch any drips); do not place the sheet pan directly under the muffin pan on the same rack or it will interfere with baking. Bake the buns for 15 minutes, then carefully press down on the tops of the buns again with a spatula. Rotate the pan and continue baking until the buns are golden brown, 10 to 15 minutes more.
Step 8 While the buns are baking, fill a pie plate or bowl with the remaining 1/2 cup [100 g] of granulated sugar. Remove the pan from the oven and immediately flip the buns onto the prepared pan. (If the centers of your buns are much lighter in color than the bottoms, you can put buns back in the oven (on the sheet pan with the wire rack) for 5 or 6 minutes until golden.) Using tongs, pick up one bun at a time and evenly coat it in the bowl of sugar. Repeat with the remaining buns, placing them on a wire rack to cool. Morning buns are best eaten the same day they are made.
