Preheat the oven to 350°F (180°C) and line the muffin pan with 12 cupcake liners. Set aside.
To make vegan buttermilk, combine unsweetened almond milk and apple cider vinegar (or lemon juice). Mix well and set aside.
In a medium-size bowl, using a whisk, combine all-purpose flour, cornstarch, baking powder, baking soda, and a pinch of sea salt. Set aside.
To a large bowl, add granulated sugar, sunflower oil, and vegan yogurt, and whisk to combine. Add vegan buttermilk and mix until incorporated. Sift in dry ingredients and mix until just combined. Then mix in crushed Biscoff cookies.
Divide the batter evenly into 12 cupcake liners. Bake in preheated oven for 20 minutes or until a toothpick inserted into the middle comes out clean. Transfer to a cooling rack and allow to cool completely before frosting.
Add soft vegan butter to a large bowl (alternatively, add the vegan butter to a stand mixer with the whisk attachment) and cream with a hand mixer on high speed until fluffy and lighter in color for about 2-3 minutes. Sift in powdered sugar, and mix until combined. Add in Biscoff spread and mix until fully combined. While mixing, add one tablespoon at a time of almond milk and mix until incorporated and fluffy.
Use an apple corer or a knife to remove the center of the cupcakes, almost reaching the base of them. Fill each cupcake with melted Biscoff spread (to make it easier, add the Biscoff spread to squeeze bottle or piping bag).
Using a piping bag with a star tip, pipe a swirl of Biscoff buttercream onto each cupcake.
Top with crushed Biscoff cookies and a drizzle of melted Biscoff spread.
Serve immediately or store them in the fridge in an airtight container.