Special equipment: a cherry pitter
For the pie crust: Cut the butter into tiny pieces and toss into a bowl with the flour and salt. Place in the freezer for 10 minutes to cool. Dump into a food processor with 3 tablespoons of ice water and pulse until the dough resembles wet sand that holds together, adding more water a spoonful at a time if necessary. Place the dough onto a long sheet of plastic wrap and shape into a disc, then wrap well and refrigerate for 30 minutes.
On a lightly floured surface, roll the dough to a 1/8-inch thickness and place into a 9-inch pie pan. Use a fork to prick all over the sides and bottom, then place in the freezer until hard, at least 20 minutes.
Preheat the oven to 400 degrees F. Cut an 18-inch sheet of aluminum foil and press it firmly into the pie shell, loosely tenting the foil around the edges of the crust. Fill with pie weights, dried beans, sugar or raw rice and bake for 20 minutes. Remove the weights by gently lifting the foil. Brush the pie crust well with the egg wash and return to the oven until the egg wash is set and the crust is golden brown, 3 to 5 minutes. Let cool completely and reduce the oven to 350 degrees F.
For the pie filling: Pit the cherries and place them in an even layer in a roasting pan. Sprinkle with the salt and roast for 30 minutes.
Meanwhile, place the Dr. Pepper and cinnamon stick in a large saucepan and bring to a boil. Reduce the heat to medium and cook until reduced by half.
Place the cold water in a small bowl and sprinkle the gelatin over evenly. Allow to soften for 5 minutes. Add to the warm Dr. Pepper reduction along with the vanilla and whisk well to combine.
Pack the cherries into the pie crust and pour the Dr. Pepper mixture on top. Refrigerate until set, at least 4 hours.
To serve: Top the pie with whipped cream and sprinkle the popping candies over the top. Serve immediately.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)