In a large bowl, whisk together cake flour, ground Biscoff cookies, ground cinnamon, baking powder, and kosher salt. Set aside.
Place sugar and egg yolks in the bowl of a stand mixer fitted with paddle attachment. Beat on medium to medium-high speed. Add vanilla extract, and heavy cream, reserving the rest. Mix on low speed until just combined.
Carefully add the flour to the wet ingredients, then gently fold to combine. Transfer to a large mixing bowl, then clean out the stand mixer bowl.
In three batches, gently fold the egg whites into the cake batter until fully incorporated.
Pour the cake batter to the prepared baking dish, then use an offset spatula to smooth into an even layer. Transfer to the preheated oven and bake until a cake tester inserted into the center of the cake comes out clean.
As soon as the cake comes out of the oven, use a wooden skewer (or fork) to poke holes all over the top of the cake. Pour the milk mixture evenly over the top. Cool to room temperature.
When ready to serve, combine heavy cream, powdered sugar, and ground cinnamon in the bowl of a stand mixer fitted with whisk attachment. Whip until medium or stiff peaks form. Spread into an even layer on top of the cake, then garnish with crushed Biscoff cookies. Enjoy!