How to Make It
Make the cake In a medium bowl, whisk the flour, baking powder, salt and cinnamon to combine.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and granulated sugar on moderate speed until smooth, 1 to 2 minutes. Scrape down the sides of the bowl. Beat in the shortening until combined. Beat in the whole eggs and yolk, one at a time, allowing each to fully incorporate before adding the next. Beat in the vanilla. Scrape down the sides of the bowl then beat just until evenly combined, 5 to 10 seconds.
With the mixer on low, gradually beat in half of the dry ingredients, until just combined, then slowly beat in the buttermilk. Add the remaining dry ingredients and the Biscoff cookies. Mix on moderate speed until combined.
Preheat the oven to 350°. Spray three 8-inch baking pans with cooking spray and line them with parchment, then spray the parchment. Divide the batter evenly between the pans, roughly 2 1/4 cups of batter per pan. Bake until golden brown and a toothpick inserted in the center comes out clean, 25 to 35 minutes. Let the cakes cool in the pans for 20 minutes, then invert onto wire racks, peel off the parchment, and let cool completely, about 1 hour. Wrap in plastic and refrigerate until ready to use.
Make the buttercream In a small pot over moderately high heat, simmer 1 1/3 cups of the granulated sugar with the water slowly until the mixture reaches 245°.
Meanwhile, in the bowl of a stand mixer fitted with a whisk attachment, whip the egg whites on high speed. Slowly pour in the remaining 2/3 cup granulated sugar until soft peaks form. With the mixer on high speed, slowly pour the sugar syrup down the sides of the bowl and whip until fluffy and slightly cooled, reaching 100 to 105°, 7 to 10 minutes.
At moderate speed, beat in the butter a few tablespoons at a time, until fully incorporated and the mixture is fluffy. Add the confectioners’ sugar, salt, vanilla and 1/4 cup of cookie butter, and whip until fully incorporated. Use right away or cover and refrigerate for up to 1 week.
Assemble the cake Transfer the buttercream to a pastry bag fitted with a 1/2-inch tip. Using a serrated knife, trim the domed tops of the cakes to flatten them; reserve the scraps for another use. Place a 10 to 12-inch plate or cake board onto a rotating cake stand or wheel. Place the first layer of cake in the center of the board. Pipe a wall of buttercream around the edge of the cake. Spread 1/4 cup of the cookie butter evenly in the center of the cake, then sprinkle with half of the Biscoff cookie crumbles. Top with more buttercream and spread evenly over the top. Repeat with the next layer of cake. Place the last cake on top, and spread a thin layer of buttercream over the entire cake as a crumb coat. Refrigerate for at least 1 hour.
Remove the cake from the fridge and coat with remaining buttercream. Serve.
Speculoos Creamy or Crunchy Cookie Butter may be purchased from Trader Joe’s. Biscoff Cookie Butter may be purchased from amazon.com.