Make the Crumble Topping: In a small bowl, combine ½ cup flour, ⅓ cup brown sugar, and ½ teaspoon cinnamon. Add 4 tablespoons melted Biscoff spread and stir with a fork until large crumbs form. Chill in the refrigerator while you prepare the muffin batter.
Preheat and Prepare the Muffin Tin: Preheat the oven to 350°F (175°C). Line a 12-hole muffin tin with paper muffin liners or grease lightly.
Mix Dry Ingredients for Muffins: In a medium bowl, whisk together 1 ½ cups flour, 1 teaspoon baking powder, ½ teaspoon cinnamon, and ¼ teaspoon salt. Set aside.
Mix Wet Ingredients: In a separate large bowl, whisk together 2 eggs and 1 teaspoon vanilla extract until bubbly. Add ⅓ cup brown sugar, ½ cup melted Biscoff spread, and ½ cup buttermilk. Mix until smooth.
Combine Wet and Dry Ingredients: Add the dry flour mixture to the wet mixture and stir gently with a silicone spatula until just combined. Fold in crushed Biscoff cookies if using.
Assemble the Muffins: Divide the muffin batter evenly between the muffin cups, filling each about ⅔ full. Sprinkle the chilled crumble topping evenly over the batter.
Bake the Muffins: Bake at 350°F (175°C) for 17-20 minutes, or until golden on top. Test for doneness by inserting a toothpick into the center of a muffin—if it comes out clean with a few moist crumbs, the muffins are done.
Cool and Drizzle: Let the muffins cool in the pan for 5-10 minutes, then transfer them to a wire rack. Drizzle with melted Biscoff spread for extra flavor.