Heat the oven to 350°F (175°C) and grease a 9 x 13-inch baking pan. Prepare the cake mix with the eggs, vegetable oil, and water according to the package directions, using the liquid and egg amounts printed on the box if they differ.

Spread the batter evenly in the prepared pan and bake for approximately 25–30 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for about 10–15 minutes.

Use the handle of a wooden spoon or the end of a large straw to make evenly spaced holes across the cake. Make the holes fairly deep so the filling can travel through the crumb, but do not pierce completely through the bottom.

Warm 1 cup of Biscoff spread in the microwave in short intervals, stirring between each interval, until it becomes fluid. Pour the sweetened condensed milk over the cake, then spread or drizzle the warm cookie butter over the surface.

Allow the cake to cool completely, then spread the thawed whipped topping over the entire surface. Sprinkle with 10–12 crushed Biscoff cookies, warm the remaining 2–3 tablespoons of spread, and drizzle it over the top.

Refrigerate the finished cake for at least 1 hour, preferably 2 hours, before slicing. Serve cold for the cleanest pieces and the best filling texture.