Make the Pie Crust: In a food processor, pulse together the flour, sugar, and salt. Add the cubed butter and pulse until butter is broken up but still coarse. Add the sour cream and continue pulsing until the dough starts to come together. Turn out onto a lightly floured surface and press the dough together into a flattened rectangle. Cover with plastic wrap and refrigerate for at least 1 hour, or overnight.

Make Cookie Butter Filling: Place butter in a saucepan over medium heat. Stir often until butter foams, then browns, about 5-7 minutes. Remove from heat. In a food processor, blend the Biscoff cookies until very finely ground. Add the browned butter, brown sugar, salt, cinnamon, and vanilla extract and pulse until mixture is combined. With the food processor running, drizzle in the heavy cream.

Make the Glaze: In a medium bowl, mix the cream cheese and powdered sugar with a spatula. Add the vanilla, heavy cream, and salt and stir to combine.

Assemble & Bake: Preheat oven to 375°F and line two half-sheet trays with parchment paper. On a lightly floured surface, roll out the chilled dough into a 12” x 18” rectangle. Trim the edges to make them straight. Using a pastry wheel, cut 4 even strips along the length (4.5” each), and 4 even strips along the width (3” each). You should have 16 rectangles. If the dough is getting soft, transfer to the sheet trays and refrigerate for 15 minutes. Put 2 ½-3 tbsp of filling in the center of half of the rectangles. Press down the filling with the back of a spoon or offset spatula, leaving a small border around the edges. Lightly brush the edges of the filled rectangles with egg wash. Top with the remaining rectangles, pressing down to adhere the edges of the dough together. Crimp the border with a fork to seal together. Cut an X in the top and brush with egg wash. Bake for 35-40 minutes until golden brown. If baking on different racks in your oven, rotate the pans after 20 minutes. Allow the pop tarts to cool, then spoon the icing on top, spreading with an offset spatula. Top with extra crushed Biscoff cookies if desired.