Preheat oven to 350 degrees. Line a muffin tin with paper liners. Set aside.
In a medium size mixing bowl, add flour, salt and baking soda, whisk to combine. Set aside.
In the bowl of a stand mixer, beat together butter, sugars, sour cream and Biscoff spread. With mixing speed on low, add eggs one at a time until incorporated. Add milk, vanilla, cinnamon, mix.
Add dry ingredients just until combined, about 30 seconds. Remove bowl from stand and fold in cookie pieces.
Scoop batter into prepared pan, filling each liner ¾ of the way full. Place in oven and bake for 18-22 minutes or until a toothpick inserted into the center comes out clean. Remove and place on a wire rack to cool for 10 minutes. Remove cupcakes from pan and allow to cool completely (25-30 minutes).
While the cupcakes are cooling, prepare buttercream frosting. In the bowl of a stand mixer, beat the softened butter and biscoff spread on medium speed until light and fluffy. Add vanilla extract and mix well.
Turn speed to low and being to add the powdered sugar (it will look like bread dough). Once the powdered sugar is added, pour the milk a little bit at a time, while mixer is on. Mix until combined. Add milk until the frosting reaches desired consistency (it should be rather stiff). Beat on medium-high speed for 4-6 minutes until fluffy. Frost cooled cupcakes as desired.
Notes
* I was unable to find creamy at my local supermarkets, so I used crunchy. Only the texture of the frosting will change if using crunchy.