In a large mixing bowl, cream together softened butter, brown sugar, and granulated sugar using a stand mixer on medium speed until light and fluffy, about 3-5 minutes. Add the Biscoff cookie butter and mix until fully combined. Incorporate the egg and egg yolk one at a time, then stir in vanilla extract for an aromatic touch.

Sift all-purpose flour, cornstarch, salt, ground cinnamon, baking powder, and baking soda over the wet ingredients. Using a spatula, gently fold the dry ingredients into the wet mixture until just combined. Stir in the white chocolate chips and crushed Biscoff cookies.

Use an ice cream scoop to form large, uniform balls of cookie dough and place them on a plate. Cover with plastic wrap and chill in the refrigerator for 4-6 hours.

Preheat your oven to 375°F (190°C). Line baking sheets with parchment paper. Arrange the chilled dough balls on prepared sheets and bake for 9-12 minutes until edges are golden; let cool for 10 minutes.

In a mixing bowl, beat softened butter until creamy, around 2 minutes. Gradually add powdered sugar, mixing slowly. Incorporate Biscoff cookie butter, vanilla extract, ground cinnamon, and salt until smooth.

Once the cookies have cooled, apply the buttercream frosting atop each cookie using a spatula or piping bag. Sprinkle crushed Biscoff cookies on top for presentation.