Preheat oven to 350F.
Prepare a 9" x 13" sheet pan by lining it with parchment paper, set aside.
Crush vanilla oreo cookies into a fine powder (a food processor works best for this). Set aside.
Cream the butter, granulated sugar, and brown sugar until combined. Add in the egg and the vanilla butter emulsion and mix again.
Add in the flour, oreo crumbs, salt, baking soda, and baking powder and mix just until the flour mixture disappears.
Using a medium cookie scoop, scoop out 16 equally sized cookie dough balls, roll them between the palm of your hands then roll them in coarse sanding sugar.
Place the cookie dough on the prepared baking sheet. Bake in the preheated oven for 15 minutes, pull the cookies out, make an indentation in the center using a ¼ cup measuring cup or the bottom of a glass. Let the cookies cool down on the baking sheet for 15 minutes then transfer to a cooling rack.
Prepare the Pastry cream mix. Add instant mix to milk. Mix on low until the powder is dissolved and then on high for 3 minutes until smooth and shiny. Let set for 10 minutes prior to filling cookies. Prepare chocolate glaze while the pastry cream is setting up.
Prepare chocolate glaze. In a microwave safe bowl, microwave dark chocolate and coconut oil for 30 seconds. Stir, and microwave for 20 seconds. Continue 20 second intervals until fluid.
Fill Cookies: Using a spoon, spoon about 1 tablespoon of pastry cream into the cookie. Top with chocolate glaze and white chocolate curls.
Storage: Cookies should be refrigerated.