Sure, here's my disorganized recipe from hell, cobbled together from various websites and experiments. I just realized that I pasted the caramel and pastry cream in the wrong order... It's easier if you make the pastry cream first and clean that pot for the caramel. I'm on mobile. Will fix.
1 box white cake mix, using egg whites. Bake. Flip onto a wire rack.
2/3 box spice cake mix (save the rest to turn into batter later, or portion the batter and bake it on the side.) I add extra allspice. Bake in previous pan.
⅓ cup butter, salted or unsalted
White chocolate and hot milk ganache (pre-prepare or don't; pour hot milk over the chocolate and stir until thick but fluid consistency. Use the same method over dark chocolate to make a separate, thicker ganache for later).
Heat until fully melted and amber, without burning. People say don't stir the sugar; I don't think it matters much for a cake like this and I stir it to help it melt faster after it has started.
Add the butter pieces and stir until smooth. It's gonna look scary. Just keep going.
Add the white chocolate ganache until reaching desired consistency. This is done now. Transfer to a holding container and immediately wash your pot. Filling the pot with water and letting the leftover scuz simmer on low helps.
Pastry Cream (it's just custard with butter, apparently) (ALSO NOTE: I never have a ton unless I make a thin tall cake, but this amount offers an appropriate texture. Maybe double this exclusively if you can make it look pretty. IMO, I'd turn this into a mousse to stabilize it, but I haven't. Spicetrekkers Walnut Espresso Blondie has a good custard->mousse setup.)
▢ 2 tablespoons unsalted butter
▢ 1 teaspoon vanilla extract (or vanilla bean, split open, to add to the milk during heating)
In a medium saucepan, combine milk and granulated sugar. Gentle simmer/steam. Do not boil.
Whisk together 1 large egg, 2 large egg yolks, ¼ cup cornstarch, and ⅓ cup granulated sugar in a mixing bowl until smooth and pale.
Slowly pour about half of the hot milk into the egg mixture in a steady stream, whisking constantly to temper the eggs and prevent curdling.
Immediately pour the tempered egg mixture back into the saucepan. Return to medium heat and whisk constantly until the mixture thickens and begins to bubble, about 2 to 3 minutes.
Remove from heat. Stir in 2 tablespoons unsalted butter and 1 teaspoon vanilla extract until fully melted and incorporated.
Transfer the pastry cream to a clean bowl. Press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate until completely chilled (can use freezer initially, if you want).
Layer as per image. Layer as follows:
Spice cake → Caramel ganache → White cake → Plain pastry cream → Pastry cream with add-ins → Dark chocolate ganache with oreos if you want it that way → Crushed biscoff cookies
It is very reminiscent of a typical Southern dump-cake, but if you organized it. It's fun. With work, it could be kind of classy?
If I had enough free-time, I'd change a lot about the layering. The caramel and spicecake is fantastic, but the pastry cream doesn't layer perfectly with the white cake. If it was up to future-me, I'd make round cakes, and stack the white in 2-3 layers with the caramelly cookie cream, and coat the entire thing in chocolate ganache (with a little espresso powder). This would do great with coconut and/or almond in a form of preference, too.
