Preheat your oven to 325 degrees F.
In a medium-size bowl, combine the Biscoff crumbs, all-purpose flour, cocoa powder, baking powder, baking soda, and salt. Whisk until the ingredients are well-blended. Set aside.
In the bowl of a stand mixer fitted the paddle attachment, beat the butter, granulated sugar and brown sugar on medium speed until light and fluffy, about 2 to 3 minutes.
Add the eggs, one at a time, beating well after each addition, making sure to scrape down the bowl between additions. Continue to beat the mixture for a few more minutes, until the mixture is smooth. It’s not done until it looks cohesive, meaning, it should not look curdled. We want it to be smooth.
With the mixer on low speed, add 1/3 of the dry ingredients and mix until combined. Slowly add half of the buttermilk and mix until incorporated. Continue alternating the flour mixture and buttermilk until it has all been incorporated. Scrape down the sides of the bowl and slowly mix for another 20-30 seconds.
Pour the batter onto the Biscoff crust in each pan (about 18 ounces in each) and bake for 30 to 35 minutes, or until a toothpick inserted in the center of the cake comes out with a few moist crumbs on it. Cool 10 minutes on a wire rack before inverting onto a cooling rack to cool completely.