First,make the rhubarb jam. Combine all ingredients in a small sauce pot and cook over medium low for about 10 minutes or until thick. Stir frequently to avoid burning. Pour into a bowl and refrigerate until needed. When jam has cooled down enough, transfer to a pastry bag.
To make the pastry dough, combine all ingredients into a food processor and combine until dough comes together in a ball. Remove and wrap in plastic. Refrigerate for 30 minutes.
To make cream cheese filling, mix cream cheese and sifted powdered sugar in a small bowl until smooth. Add salt and vanilla and mix until fully incorporated and smooth. Transfer to a pastry bag and set aside.
Mix egg wash together in a small bowl by whisking egg and water together. Set aside.Roll out pastry dough on a lightly floured work surface to 1/8” thickness. Cut 2x3-inch rectangles, as many as you can. You can re roll out the dough once to get a few more rectangles. You should get 10 rectangles total.
Line a sheet tray with parchment paper and place 5 rectangles of dough 1 ½ inches apart from each other. The pastry will not spread during the bake, so it doesn’t need too much space. Use a pastry brush to brush the entire rectangle of dough. Pipe straight lines of rhubarb jam, followed by cream cheese, ½ inch away from the edges. Reserve 1 T of jam to create the glaze later. Continue to do with all of the bottom pieces of dough. Carefully cover the bottom with a rectangle top. Use a fork to press down and crimp sides. Continue with remaining pieces of dough. Brush the top of the pop tart with egg wash. Freeze pop tarts while preheating the oven.
Preheat the oven to 350 degrees F. Use a pairing knife to cut 2 slits into the center of the pop tarts to release steam while baking.Bake for 24 minutes total, rotating the pan half way through.
Make glaze by mixing sifted powdered sugar, corn syrup, and reserved jam. Whisk together and add 1 tsp of milk as needed to create the consistency you like. Set aside.
Once pop tarts are baked, cool down at room temperature for about 30 minutes. Use a spoon or offset spatula to smooth over the rhubarb glaze. Let it cool completely at room temperature until glaze is set, about 1 hour.