Preheat the oven to 400° F and line a small baking sheet with parchment paper.

Combine the flour, and butter using an electric mixer or food processor. The mixture will look like little lumps or crumbles. Add about 1/3 cup of cold water, little by little, until the dough starts to come together. If it is dry, add a little more water. If it is too sticky, add a little more flour.

On a floured surface, roll the dough out into about 1/8" thickness. Cut 8, even-sized rectangles, and then cut each rectangle in half.

In a small microwave-safe dish, melt the peanut butter and then the marshmallow Fluff for about 30 seconds or until melty. Spread a dollop of Fluff on half of the rectangles (the other 8 pieces will be the tops!) Make sure there is a border on the outside of each dough rectangle. Brush the border of each Pop-Tart with the egg wash. Then, place the tops on each of the 8 rectangles, and press the edges with a fork to seal.

Carefully transfer the pop tarts to the prepared pan and freeze for 20 minutes. Bake the pop tarts for about 18 minutes (it may be somewhere in between 15-20, depending on your oven). Cool completely.