Preheat your oven to 325 degrees F. Spray an 8×8 square inch cake pan with cooking spray, line with parchment paper, and spray again. Set aside.
Place the Biscoff cookies in a blender and blend until the cookies are finely ground into a fine crumb.
In a medium-sized bowl, whisk together the Biscoff crumbs, all-purpose flour, baking powder, baking soda, and salt until evenly combined.
In the bowl of a stand mixer fitted the paddle attachment, cream the butter, brown sugar, and oil on medium speed until light and fluffy, about 4 minutes. The mixture will start looking light in color.
Add the eggs, one at a time, beating on medium speed for 30 second after each addition. Add the vanilla and mix on the lowest speed, just until combined.
With the mixer on the lowest speed, add half of the dry ingredients and half of the buttermilk. Mix just until combined. Repeat this step. Add the rest of the flour mixture and continue to mix on the lowest speed until the flour and the buttermilk have all been incorporated. With a rubber spatula scrape down the sides and bottom of the bowl and slowly mix by hand.
Add the the batter to the prepped pan, and carefully smooth the batter with a small offset spatula and bake until a toothpick comes out clean, or with only a few crumbs on it, about 35-40 minutes. Depending on your oven.
After they are out of the oven, I let mine cool for 15 minutes. Then I transfer the cake onto a wire rack to finish cooling in the fridge or freezer to help speed up the process. (20 minutes in the freezer, 30 minutes in the fridge)
While the cake is cooling, make the frosting.
For the frosting
In a stand mixer fitted with a paddle attachment, beat the butter and Biscoff spread on medium speed for roughly 2 minutes.
Once combined, add the cream, powdered sugar, and salt and beat again on medium speed until light and fluffy, about 3 minutes.
With a rubber spatula or a wooden spoon, beat out the air pockets by hand.
Assembly
When the cake is cooled completely, using an offset spatula, evenly spread the buttercream on top of the cake. If you want the same design that I used, I use Wilton 1M piping tip.