In a large mixing bowl, whisk together the vegetable oil and egg until well combined.
Add the light brown sugar and vanilla extract, then whisk until smooth.
Pour in the milk and whisk until fully incorporated.
Add the melted Biscoff cookie butter spread and mix until smooth.
Add the self-rising flour and mix until just combined. Do not overmix.
Crush the Biscoff cookies and gently fold most of them into the batter, reserving some for the topping.
Divide the batter evenly among the paper-lined muffin cups.
Bake at 375°F (190°C) for 20–25 minutes, or until a toothpick inserted into the center comes out clean.
Transfer the melted Biscoff cookie butter to a piping bag.
Spread a little more melted Biscoff cookie butter over the top of each muffin.
Sprinkle the remaining crushed Biscoff cookies over the tops.
The Biscoff Muffins are ready to serve.